Add env_var and file as supported types for CI variables. Variables of
type file expose to users existing gitlab-runner behaviour - save
variable value into a temp file and set the path to this file in an ENV
var named after the variable key.

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/46806.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26248 added
support for deduplicating FindCommit requests using Gitaly ref name
caching. However, not all endpoints were covered, and in one case the
Gitaly wrapper wasn't actually surrounding the serialization step. We
can safely cache ref names between FindCommit calls for #index and #show
endpoints for merge requests and pipelines. This can significantly
reduce the number of FindCommit requests.

For each pipeline, the controller will call `Pipeline#latest?` to
determine if the pipeline's ref is the latest for that branch.
Since it's likely that the same branches are being used in each
pipeline, we can reduce Gitaly overhead by caching the results
of the FindCommit call.

Enables frozen string for some vestigial files as
well as the following:

* app/controllers/projects/**/*.rb
* app/controllers/sherlock/**/*.rb
* app/controllers/snippets/**/*.rb
* app/controllers/users/**/*.rb

Partially addresses #47424.

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50461

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

The status is shown for
- The author of a commit when viewing a commit
- Notes on a commit (regular/diff)
- The user that triggered a pipeline when viewing a pipeline
- The author of a merge request when viewing a merge request
- The author of notes on a merge request (regular/diff)
- The author of an issue when viewing an issue
- The author of notes on an issue
- The author of a snippet when viewing a snippet
- The author of notes on a snippet
- A user's profile page
- The list of members of a group/user

This adds Keyset pagination to GraphQL lists. PoC for that is
pipelines on merge requests and projects.

When paginating a list, the base-64 encoded id of the ordering
field (in most cases the primary key) can be passed in the `before` or
`after` GraphQL argument.

this required moving some data store actions & mutations around

render job items (needs improvements to components)

This makes it possible to avoid N+1 queries when loading pipelines
table.

When displaying a project's pipelines
(Projects::PipelinesController#index) we now exclude the coverage data.
This data was not used by the frontend, yet getting it would require one
SQL query per pipeline. These queries in turn could be quite expensive
on GitLab.com.

When displaying the pipelines of a project we now preload the following
data:

1. Authors of the commits that belong to these pipelines
2. The number of warnings per pipeline, which is used by
   Ci::Pipeline#has_warnings?

== Commit Authors

Previously this data was queried for every Commit separately, leading to
20 SQL queries being executed in the worst case. With an average of 3 to
5 milliseconds per SQL query this could result in 100 milliseconds being
spent in _just_ getting Commit authors.

To preload this data Commit#author now uses BatchLoader (through
Commit#lazy_author), and a separate module
Gitlab::Ci::Pipeline::Preloader is used to ensure all authors are loaded
before they are used.

== Number of warnings

This changes Ci::Pipeline#has_warnings? so it supports preloading of the
number of warnings per pipeline. This removes the need for executing a
COUNT(*) query for every pipeline just to see if it has any warnings or
not.

When displaying the project pipelines dashboard we display a few tabs
for different pipeline states. For every such tab we count the number of
pipelines that belong to it. For large projects such as GitLab CE this
means having to count over 80 000 rows, which can easily take between 70
and 100 milliseconds per query.

To improve this we apply a technique we already use for search results:
we limit the number of rows to count. The current limit is 1000, which
means that if more than 1000 rows are present for a state we will show
"1000+" instead of the exact number. The SQL queries used for this
perform much better than a regular COUNT, even when a project has a lot
of pipelines.

Prior to these changes we would end up running a query like this:

    SELECT COUNT(*)
    FROM ci_pipelines
    WHERE project_id = 13083
    AND status IN ('success', 'failed', 'canceled')

This would produce a plan along the lines of the following:

    Aggregate  (cost=3147.55..3147.56 rows=1 width=8) (actual time=501.413..501.413 rows=1 loops=1)
      Buffers: shared hit=17116 read=861 dirtied=2
      ->  Index Only Scan using index_ci_pipelines_on_project_id_and_ref_and_status_and_id on ci_pipelines  (cost=0.56..2984.14 rows=65364 width=0) (actual time=0.095..490.263 rows=80388 loops=1)
            Index Cond: (project_id = 13083)
            Filter: ((status)::text = ANY ('{success,failed,canceled}'::text[]))
            Rows Removed by Filter: 2894
            Heap Fetches: 353
            Buffers: shared hit=17116 read=861 dirtied=2
    Planning time: 1.409 ms
    Execution time: 501.519 ms

Using the LIMIT count technique we instead run the following query:

    SELECT COUNT(*)
    FROM (
        SELECT 1
        FROM ci_pipelines
        WHERE project_id = 13083
        AND status IN ('success', 'failed', 'canceled')
        LIMIT 1001
    ) for_count

This query produces the following plan:

    Aggregate  (cost=58.77..58.78 rows=1 width=8) (actual time=1.726..1.727 rows=1 loops=1)
      Buffers: shared hit=169 read=15
      ->  Limit  (cost=0.56..46.25 rows=1001 width=4) (actual time=0.164..1.570 rows=1001 loops=1)
            Buffers: shared hit=169 read=15
            ->  Index Only Scan using index_ci_pipelines_on_project_id_and_ref_and_status_and_id on ci_pipelines  (cost=0.56..2984.14 rows=65364 width=4) (actual time=0.162..1.426 rows=1001 loops=1)
                  Index Cond: (project_id = 13083)
                  Filter: ((status)::text = ANY ('{success,failed,canceled}'::text[]))
                  Rows Removed by Filter: 9
                  Heap Fetches: 10
                  Buffers: shared hit=169 read=15
    Planning time: 1.832 ms
    Execution time: 1.821 ms

While this query still uses a Filter for the "status" field the number
of rows that it may end up filtering (at most 1001) is small enough that
an additional index does not appear to be necessary at this time.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/43132#note_68659234
for more information.