Continues the iteration on the project overview UI:

- moved star, fork and new clone button
(copy SSH/HTTPS URLs) to top right,
  made them smaller
- avatar is now larger (64px)
- 'Request access' is now a link instead
  of a button
- overview comes before the description +
  changed styling and added icons
- description font-size is now 16px (large-paragraph)
- quick links to files are moved downwards
  below the commit/pipeline info
- margins changed to group content into
  4 groups to clean up the interface
- visibility info reduced to icon-only and
  moved to the right of the title

When a project is disabled, visiting the home page would show,
"An error occurred while loading commit signatures". This change checks
that the user has permission to view the project to avoid unnecessary
GPG signature lookups.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50903

The repository languages feature got introduced through:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19480. This MR
included feature flags to guard against performance regressions, which
allowed GitLab to test it on gitlab.com before the customers would be
exposed.

.com worked out fine, as shown by our internal monitoring tools, which
prompts feature gate removal.

Given the docs weren't present yet, these were added too.

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.