Remove `keep_divergent_refs` feature flag, adding documentation and changelog

Feature added in:

• !25639 (merged)
• !26405 (merged)
• !26406 (merged)
• !26407 (merged)

Feature expanded in:

• gitaly!1915 (merged)
• gitaly!2028 (merged)
• !29082 (merged)

Internal rollout and testing

1. See ae62fb08 through to production.
2. Enable keep_divergent_refs feature flag for projects that have a Canonical -> Security -> Build mirroring structure. Note the flag is scoped to projects, not groups.
3. Enable keep_divergent_refs for all of our push mirrors via API:

   $ http GET 'https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab/remote_mirrors/'
   [
       {
           "enabled": true,
           "id": 177728,
           "keep_divergent_refs": false,
           "last_error": null,
           "last_successful_update_at": "2020-04-28T19:35:34.695Z",
           "last_update_at": "2020-04-28T19:35:34.695Z",
           "last_update_started_at": "2020-04-28T19:36:42.044Z",
           "only_protected_branches": true,
           "update_status": "started",
           "url": "https://*****:*****@gitlab.com/gitlab-org/security/gitlab.git"
       }
   ]
   
   $ http PUT 'https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab/remote_mirrors/177728' keep_divergent_refs=true

General release

• Remove feature flag check
• Add changelog entry
• Add API documentation
• Add User documentation
• Internationalization
• Address spec nitpick from !29082 (comment 333135591)
• Delete feature flag entry via ChatOps