Add tooltips to ellipsis dropdowns in MR page

What does this MR do and why?

• Removes extra padding on right side of threads control (previously there because we didn't show the options when all threads were resolved)
• Adds tooltips to both MR actions and Threads options

Screenshots or screen recordings

How to set up and validate locally

1. Go to MR page
2. Hover over buttons at top right, tooltip should appear on ellipsis button
3. Start threads
4. Check that tooltip appears on ellipsis button when there is at least 1 unresolved thread and when all threads are resolved

Related to #373738 (closed)

Related to ✨ Beautifying our UI: Create:Code Review ✨ (#364574 - closed)