Multiple assignees review

What does this MR do?

This adds multiple assignees for issues to EE

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

Everything

Why was this MR needed?

Deliverable

Screenshots (if relevant)

To be added

Todo

• Enable multiple assignees for create issue form (FE) - !1766 (merged)
• Add tests to ^^ (FE) - !1766 (merged)
• Add tests for changes made in !1456 (merged) (FE) - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1791
• Add tests for changes made in !1731 (merged) (FE) - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1790
• Enable multiple assignees for bulk update (FE & BE) - !1804 (merged)
• Fix bug where clicking assign yourself doesn't display the selected assignee inside the assignee(s) section (FE) - !1781 (merged)
• Fix bug where user dropdown is not displaying checkmark for selected item Eg. MR edit page (FE) - !1771 (merged)
• Fix bug where avatar counter on the issue board bleeds over the edge (FE) - !1813 (merged)
• Get all tests passing (FE & BE)
• Merge master into this review branch (FE & BE)
• Create CE MR based on this review branch and limit the assignee to 1 (FE & BE)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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?

#1904 (closed)