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.

- Only if user can manage project,
- When Auto DevOps is not enabled,
- When project configuration is set to implicit

[ci skip]

This fixes an issue where the content has a limited width but the
breadcrumbs remained full width. Instead of creating a new variable
specifically for the breadcrumbs, it just re-uses @content_class

Closes #34573

Also moves action buttons into the new breadcrumbs

Closes #34030, #34033

Update show.html.haml to allow guests to see the Files tab as per https://gitlab.com/gitlab-org/gitlab-ce/issues/30199

fix lint issues