btn-small -> btn-sm

Merged Simon Knox requested to merge 3637-small-button into master

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14113 for EE

Why was this MR needed?

  • Approve button too big
  • Rebase button too big
  • Add related issue button icon misaligned

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #3637 (closed)