Replace select2 dropdowns with DropLab

Summary

We need to replace select2 dropdowns with new DropLab dropdowns for UI consistency. Please comment in this issue if you'd like to grab on one of the items from the checklist, and open a separate MR for each one!

Simple dropdowns can still be implemented in bootstrap.

Affected Pages Checklist

• app/views/admin/application_settings/_form.html.haml
• app/views/admin/groups/show.html.haml(http://localhost:3000/admin/groups/test-avatars)
• app/views/groups/milestones/new.html.haml (http://localhost:3000/groups/gitlab-org/milestones/new)
• app/views/import/github/status.html.haml (http://localhost:3000/import/github/status)
• app/views/profiles/notifications/show.html.haml (http://localhost:3000/profile/notifications)
• app/views/profiles/show.html.haml (http://localhost:3000/profile)
• app/views/projects/blob/_editor.html.haml(http://localhost:3000/gitlab-org/gitlab-ce/edit/{branch}/{file})
• app/views/projects/edit.html.haml (http://localhost:3000/gitlab-org/gitlab-ce/edit)
• app/views/projects/new.html.haml(http://localhost:3000/projects/new)
• app/views/projects/project_members/import.html.haml (http://localhost:3000/gitlab-org/gitlab-ce/project_members/import)
• app/views/shared/_new_project_item_select.html.haml
• app/views/shared/issuable/_form.html.haml

Other Links

• Similar issue (#21508 (closed)) but it was just for the todos page and was resolved with !6072 (merged)
• Screenshots of affected pages from @dimitrieh's notes