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.

- Includes new image from gitlab-svgs
- Updates dependency for svgs

Remove duplicate ee code

This was generalized for usage in the new variable table design, see
https://gitlab.com/gitlab-org/gitlab-ce/issues/39118#note_53615249

Conflicts:
	app/assets/stylesheets/pages/projects.scss
	ee/app/assets/javascripts/protected_branches/protected_branch_create.js
	ee/app/assets/javascripts/protected_tags/protected_tag_create.js
	ee/app/views/projects/protected_branches/ee/_dropdown.html.haml

* new merge request can be created by sending an email to the specific
email address (similar to creating issues by email)
* for the first iteration, source branch must be specified in the mail
subject, other merge request parameters can not be set yet
* user should enable "Receive notifications about your own activity" in
user settings to receive a notification about created merge request

Part of #32878

Add tests
Update empty state
[ci skip]