Resolve "Unnecessary padding along left side of assignees dropdown"

What does this MR do?

• Show checkmark next to assigned user in dropdown
• Vertically center checkmarks

Why was this MR needed?

• Too much padding to the left of the assignee dropdown
• Needed an indicator to show who is currently selected in the list

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?

Closes #31057 (closed)