Make merge request form dropdowns consistent

This is the same as https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13587.

What does this MR do?

Change dropdowns in the merge request form to new style. This affects new and edit merge request pages.

Screenshots

Before	After

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/28921, https://gitlab.com/gitlab-org/gitlab-ce/issues/35424

blocked by https://gitlab.com/gitlab-org/gitlab-ee/issues/3268

added blocked label

added 96 commits

• 773ade74...1a8ee80e - 95 commits from branch master
• 94793ee4 - Make merge request form dropdowns consistent

Compare with previous version

I can no longer reproduce https://gitlab.com/gitlab-org/gitlab-ee/issues/3268

removed blocked label

changed the description

added 1 commit

• 0a4fdbde - Make merge request form dropdowns consistent

Compare with previous version

@cperessini can you please approve this one, too?

assigned to @cperessini

@winh LGTM. Approving.

assigned to @winh

approved this merge request

assigned to @iamphill

merged

mentioned in commit a1befd12