Related Issues UX improvements - loading

What does this MR do?

• Move the loading icon into the panel body
• Add transition when the panel body collapses away after fetching and the loading icon goes away
• Add loading icon to the submit button
• Server-side render basic panel heading for Related Issues as a placeholder until the JS kicks-in

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

• Do we want the loading spinner to be centered in the panel body?
  • The loadingIcon Vue component has text-center in it

Why was this MR needed?

Follow-up feedback to address, https://gitlab.com/gitlab-org/gitlab-ce/issues/33533

Does this MR meet the acceptance criteria?

• 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 #33533 (closed)