Resolve "Create a serializer to render issuables sidebar"

Closes #44984

See merge request gitlab-org/gitlab-ce!23379

Also refactored issues and MRs to use the same email template file

Renders empty index page
Adds Releases entry to the navigation bar behind a feature flag
Renders 404 when feature flag for releases is not enabled

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

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

A feature spec to test this simple behavior takes about 2 minutes to run
in CI. Everything it's testing is conditionals and `href` attributes,
which can easily be done in a view spec that runs in about 8 seconds.

Improve performance of tree rendering in repositories with a lot of items

See merge request gitlab-org/gitlab-ce!16511

Rails is slow to generate paths dynamically especially when called
hundreds/thousands of times. Also, rendering many partials hundreds
of times can be slower. This change reduces the number of partials
rendered and introduces two fast path methods to speed up path
generation.