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.

Why and how to enable this is covered in the docs being changed
here. This requires gitlab-org/gitlab-shell@2e8b670 ("Add support for
SSH certificate authentication", 2018-06-14) which has been merged in
and tagged as 8.0.0, so GITLAB_SHELL_VERSION needs to be bumped.

Merging this closes gitlab-org/gitlab-ce#34572 see
gitlab-org/gitlab-shell!207 for the gitlab-shell MR.

Implementation notes:

 - The APIs being changed here are all internal, and their sole
   consumer is gitlab-shell.

 - Most of the changed code is a MR to gitlab-shell, see the
   gitlab-org/gitlab-shell!207 MR. That change covers why only some of
   the internal methods get a new "username" parameter, and why some
   others only get a "user_id".

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

This reverts merge request !20801

While reviewing this branch it was noted that this task was broken.
Given the limited value this task brings, and given the fact that much
of the old behaviour has been moved to be automatically fixed, removing
was a good option.

Suggestion in:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20806#note_91261001

These tasks are happening through housekeeping right now, by default
ever 10th push. This removes the need for these tasks.

Side note, this removes one of my first contributions to GitLab, as back
than I introduced these tasks through: 54e6c004

Closes https://gitlab.com/gitlab-org/gitaly/issues/768

This can be done trough the API for the current user, or on the
profile page.

Closes #26512

The previous implementation would hit the database each time
and provide a dummy response. If the database goes down, this
means all application workers would be taken out of service.
Simplify this check by using a Rails middleware that intercepts
this endpoint and returns a 200 response.

This is refactoring in the lead up to passing mutual TLS certs for helm applications. As such we expect all applications to need config files so we can remove the logic about which applications need and do not need this (ie `#config_map?`).

This reverts merge request !20679