Only allow group/project members to mention `@all`

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/3473

Added 45 commits:

• 672cbbff...e3befaed - 43 commits from branch master
• 396c5c97 - Fix specs
• 43a3a401 - Merge branch 'master' into mention-all

Added 1 commit:

• 9a0e16f4 - Fix spec

Status changed to merged

mentioned in commit a97a2d27

mentioned in issue #20989 (closed)