WIP: Clean up new dropdown styles

What does this MR do?

Cleans up the changes made by merge requests linked in https://gitlab.com/gitlab-org/gitlab-ce/issues/28921#note_36011920.

Why was this MR needed?

The new dropdown style was applied per page. This merge request activates it globally.

Screenshots

See linked merge requests in https://gitlab.com/gitlab-org/gitlab-ce/issues/28921#note_36011920.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28921

added 34 commits

• fc5af19c...76499825 - 32 commits from branch master
• d5dd7da3 - Reuse FontAwesome icons for dropdown instead of copying
• e7ee47a2 - Make new dropdown style global

Compare with previous version

added 107 commits

• e7ee47a2...646aae3e - 105 commits from branch master
• f9101432 - Reuse FontAwesome icons for dropdown instead of copying
• 05be8e19 - Make new dropdown style global

Compare with previous version

mentioned in merge request !12388 (merged)

mentioned in merge request !13616 (merged)

added 206 commits

• 05be8e19...676f7726 - 204 commits from branch master
• 6968fe71 - Reuse FontAwesome icons for dropdown instead of copying
• c442f4ec - Make new dropdown style global

Compare with previous version

added 211 commits

• c442f4ec...7839a5ec - 209 commits from branch master
• 769f8d48 - Reuse FontAwesome icons for dropdown instead of copying
• b7526f1f - Make new dropdown style global

Compare with previous version

mentioned in merge request !13626 (merged)

mentioned in merge request !13628 (merged)

mentioned in merge request !14030 (merged)

changed milestone to %10.1

mentioned in issue #28921 (moved)

mentioned in commit dinsaw/gitlab-ce@ab0d5e27