Improve form spec (EE)

What does this MR do?

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11879

This breaks out the EE spec from the CE spec to minimize conflicts during CE => EE.

CE has single assignee (and it autocloses after assignee is selected) EE has multiple assignees (and it does not autoclose after selection)

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

Why was this MR needed?

Screenshots (if relevant)

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?