Add loading button

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3165

Add loading button

Encompasses designs from

• https://gitlab.com/gitlab-org/gitlab-ce/issues/23206#note_43159858
• https://gitlab.com/gitlab-org/gitlab-ce/issues/29985#note_3041740

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

• Snippet patch diff to try it out: https://gitlab.com/snippets/1680037

Why was this MR needed?

Going to be used in asynchronous participants Deliverable, https://gitlab.com/gitlab-org/gitlab-ce/issues/23206

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?