Merge branch 'developers-can-merge' into 'master'

Fix conflicts for the "developers can merge" feature

- This feature was implemented for CE in gitlab-org/gitlab-ce!4892.
- It was reverted in CE-master because of merge conflicts with EE-master
- This MR fixes the conflicts. Once this is merged in, the feature can be reintroduced to CE-master

/cc @rymai @rspeicher 

See merge request !564