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>

Since the frontend for this feature isn't ready, better to hide the
confusing field behind a feature flag.

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 reverts merge request !20679

Backports relevant changes made in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5163 to CE

We also try to unify the way we setup OmniAuth, and how we check
if it's enabled or not.

This method is unneeded given its been pushed to Gitaly, through
https://gitlab.com/gitlab-org/gitaly/merge_requests/780

Closes https://gitlab.com/gitlab-org/gitaly/issues/1235

This reverts commit 4d9a3f42, reversing
changes made to ecf9c145.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>