Added comment for `key` usage

Using strong params to require the presence of a username when calling
`update_username`. Otherwise we'd raise a `NoMethodError` validating
the paths on disk.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Closes gitaly#1110

This was sending the current user, which is the recipient! It should be the user
who pushed the commits.

Whenever cache is enabled and cold, branches index still makes
n+1 calls. Make sure we catch this.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

These changes are backported from EE, related to SAML settings in
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4549

Also reorganize application settings related to repository in more maninful sections
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

In UpdatePagesService, we were seeing a large number of open pack files in
production. Calling Project#cleanup removes a reference to Rugged::Repository
and forces libgit2 to close file descriptors and free its internal memory
cache.

Relates to https://gitlab.com/gitlab-com/infrastructure/issues/3965#note_65837525

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

The purpose of this memoization is to make getting persisted environment
name, and related scoped variables, a little more performant task,
because it can be invoked multiple times.

A label name can contain a `_`, `~~`, or other Markdown-significant
characters. But label references are processed _after_ Markdown processing has
run, so we can't easily fix this on the backend.

We can make it more convenient, though, by changing the frontend to escape these
characters so they aren't processed as Markdown, when we insert them from
autocomplete.

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