WIP: Add smooth expand/collapse transition to the loading button

What does this MR do?

Add smooth expand/collapse transition to the loading button

Previous MR, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14883

.	Before (fade)	After (expand-fade)
loading
indeterminate

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

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

Why was this MR needed?

Designs showed smooth expand/collapse but our first iteration was just fading things, see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14883#note_44109014

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?