Add GFM support to nested groups

What does this MR do?

• Use new regex for user reference parsing that supports / in path
• Allows mention of nested groups in gfm like @gitlab/subgroup

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

no

Why was this MR needed?

So you can mention nested group in gfm like @gitlab/subgroup

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support 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