Resolve "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.

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

Just make sure architecture is good

Why was this MR needed?

Screenshots (if relevant)

Before	After

Before	After

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)