Revert "Revert "Merge branch '18193-developers-can-merge' into 'master'

CHANGELOG

app/assets/javascripts/protected_branches.js.coffee

app/controllers/projects/protected_branches_controller.rb

app/helpers/branches_helper.rb

app/models/merge_request.rb

app/models/project.rb

app/models/repository.rb

app/services/commits/change_service.rb

app/services/files/base_service.rb

app/services/git_push_service.rb

app/services/merge_requests/merge_service.rb

app/services/merge_requests/refresh_service.rb

.../projects/merge_requests/widget/open/_conflicts.html.haml

...iews/projects/protected_branches/_branches_list.html.haml

...s/projects/protected_branches/_protected_branch.html.haml

app/views/projects/protected_branches/index.html.haml

...9203051_add_developers_can_merge_to_protected_branches.rb

..._column_in_progress_merge_commit_sha_to_merge_requests.rb

db/schema.rb

lib/api/helpers.rb