Extract parts of LegacyDiffNote into DiffOnNote concern and move part of responsibility to other classes

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

Rename ENV['PROTOCOL'] to ENV['GL_PROTOCOL'] to conform to what GitLab Shell expects and make the `protocol` param in `GitAccess` mandatory.

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

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

`avatar.present?` goes through CarrierWave, and checks that the file
exists on disk and checks its filesize. Because we're hitting the disk,
this adds extra overhead to something where the worst-case scenario is
rendering a broken image.

Instead, we now just check that the _database attribute_ is present,
which is good enough for our purposes.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/19273

- Group / project members cannot request access
- Group members cannot request access to a group's project

This addresses an issue where project owners could request access
to their own project, leading to UI inconsistency where their requester
status would replace their owner status.
Signed-off-by: NRémy Coutable <remy@rymai.me>

Feedback from:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4964#note_12867273

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

And create new Project#requesters, Group#requesters scopes.
Signed-off-by: NRémy Coutable <remy@rymai.me>

When pushing commits to existing branches we don’t
need to flush branch git data (branch names / counts)

When flushes the cache when pushing commits skip to
flush branch and tag git data (names / counts) because
those operations are managed explicitly in each case

Repopulated expired cache as soon as possible

There's little point to cut that down.

This method now should return false if build is not completed. If build
is running then it is not retryable, therefore `Ci::Build#retryable?`
returned wrong value. This commit changes this behavior