Assignee dropdown does not display selected assignee

What does this MR do?

Fixes 2 terrible bugs in multiple assignees dropdown.

1. Fixes a bug whereby the selected assignee would not show up on the Assignee(s) section because the assignee is not returned in the users api call due to pagination.
2. Fixes a bug whereby the dropdown is left in a weird state after the dropdown is dismissed after a search is performed.> Backport from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1968

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

Shouldn't be

Why was this MR needed?

regression

Screenshots (if relevant)

See screenshots in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1968

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?

Closes #2472 (closed)