Building on support for setting feature flags by project, this adds
support for setting them by GitLab group path.

This is different from setting them by Flipper feature_groups, which
are for batch updating pre-registered collections.

For features the feature gates are sometimes projects, not groups or
users. For example for git object pools:
https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5872

This commit allows for setting feature group gates based on projects, by its
path as that seems most convenient.

Enables frozen for the following:

* lib/*.rb
* lib/banzai/**/*.rb
* lib/bitbucket/**/*.rb
* lib/constraints/**/*.rb
* lib/container_registry/**/*.rb
* lib/declarative_policy/**/*.rb

Partially addresses #47424.

* Fix typo in context 'when request store is active'
* Rearrange test since the instance variable always gets set now,
even if RequestStore is active

These are clear wins.

This allows you to default a feature flag to 'on' when
checking whether it's enabled/disabled.

Previous code would not work with `disabled?` because that method would
send two parameters (second always `nil`) which we are not mocking.

Instead of mock yet another state, I decide to fix it where it belongs.

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.