WIP Add GFM support to nested groups (refactoring concept)

What does this MR do?

• Extract group reference match into own filter: GroupReferenceFilter
• Add support to nested groups in GFM

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

To make @foo/bar subgroup mention works.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/2772

mentioned in issue #2772 (closed)

changed milestone to %9.0

@DouweM as GitLab Flavored Markdown expert what do you think about separating Group into own filter?

The reason for this is different regex for usernames and groups (which can contain / now) and as result different lookup. To avoid more if/else I believe its better long-term to split those.

added 1 commit

• cf981a23 - Add GFM support to nested groups

Compare with previous version

added 1 commit

• 45177e78 - Add GFM support to nested groups

Compare with previous version

I will open a separate MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9184) where will be only modifications to user filter without refactoring/decouple so we can get it faster while this MR is being discussed/refactored

changed title from WIP Add GFM support to nested groups to WIP Add GFM support to nested groups (refactoring concept)

changed milestone to %9.1

mentioned in merge request !9184 (merged)

as GitLab Flavored Markdown expert what do you think about separating Group into own filter?

@dzaporozhets Sounds reasonable

mentioned in issue #30788 (moved)

Not working on this right now. I created an issue with technical dept for it https://gitlab.com/gitlab-org/gitlab-ce/issues/30788

closed