Our friends at GitHub show the programming languages for a long time,
and inspired by that this commit means to create about the same
functionality.

Language detection is done through Linguist, as before, where the
difference is that we cache the result in the database. Also, Gitaly can
incrementaly scan a repository. This is done through a shell out, which
creates overhead of about 3s each run. For now this won't be improved.

Scans are triggered by pushed to the default branch, usually `master`.
However, one exception to this rule the charts page. If we're requesting
this expensive data anyway, we just cache it in the database.

Edge cases where there is no repository, or its empty are caught in the
Repository model. This makes use of Redis caching, which is probably
already loaded.

The added model is called RepositoryLanguage, which will make it harder
if/when GitLab supports multiple repositories per project. However, for
now I think this shouldn't be a concern. Also, Language could be
confused with the i18n languages and felt like the current name was
suiteable too.

Design of the Project#Show page is done with help from @dimitrieh. This
change is not visible to the end user unless detections are done.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Also, don't use the provided Middleware, which isn't thread-safe, and
instantiate a new Flipper instance per thread instead.

Signed-off-by: Rémy Coutable <remy@rymai.me>

- Added an ActiveSupport (using Rails.cache) caching adapter
- Overview of the new features can be found at https://johnnunemaker.com/flippin-features-at-runtime/
- Full Changelog can be found at https://github.com/jnunemaker/flipper/blob/v0.11.0/Changelog.md



Signed-off-by: Rémy Coutable <remy@rymai.me>

The GitHub importer (and probably other parts of our code) ends up
calling Feature.persisted? many times (via Gitaly). By storing this data
in RequestStore we can save ourselves _a lot_ of database queries.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/39361

This is a follow-up for !12362 where this was documented but the code
was removed in the last iteration. Since this can still be useful and
this is already supported by the API, I think re-adding the code was
the best course of action.

Signed-off-by: Rémy Coutable <remy@rymai.me>

The implementation now simply rely on the
`performance_bar_allowed_group_id` Application Setting.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

A `performance_team` Flipper group has been created. By default this
group is nil but this can be customized in `gitlab.yml` via the
performance_bar.allowed_group setting.

Signed-off-by: Rémy Coutable <remy@rymai.me>

This allows to enable/disable a feature flag for a given user, or a
given Flipper group (must be declared statically in the `flipper.rb`
initializer beforehand).

Signed-off-by: Rémy Coutable <remy@rymai.me>

This ensures we make maximum one call per feature per request.

See
https://github.com/jnunemaker/flipper/blob/v0.10.2/docs/Optimization.md#memoizing-middleware



Signed-off-by: Rémy Coutable <remy@rymai.me>

This is controlled with the feature flag gitlab_git_diff_size_limit_increase.
Both of these limits were basically picked arbitrarily in the first place;
disabling the feature flag reverts to the old limits.