MergeRequest#all_pipelines fetches Ci::Pipeline records from the source
project, so we should specifically check that project for permissions.
This was already happening for intra-project merge requests, but in the
event that the target and source projects both have private builds, we
should ensure that the project permissions are respected.

This commits extracts /merge_requests/1.json?serializer=widget
Into a separate /merge_requests/1/widget.json endpoint
This will allow to use caching for this request

This MR is a backport of an EE merge request

Backports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161
(code out of ee/ folder).

Allow single tasks to be updated quickly

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.

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

"Maintainer" will be freed to be used for #42751

When a project is not private, and the source branch not protected the
user can now select the option to allow maintainers to push to this
branch

add a spec for commit merge request diff notes

also, I refactored the MergeRequest#fetch_ref method to express
the side-effect that this method has.

  MergeRequest#fetch_ref -> MergeRequest#fetch_ref!
  Repository#fetch_source_branch -> Repository#fetch_source_branch!

In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.

Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.

Closes gitlab-org/gitlab-ce#37534.

Removed merge_request/issue state_event param parent and removed issuable_path helpers in favour of issuable_url helpers