Backport some changes done from Time Tracking feature in EE.

app/controllers/concerns/issuable_actions.rb

app/controllers/projects/issues_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/controllers/projects/notes_controller.rb

app/models/concerns/issuable.rb

app/serializers/issuable_entity.rb

app/serializers/issue_entity.rb

app/serializers/issue_serializer.rb

app/serializers/label_entity.rb

app/serializers/merge_request_entity.rb

app/serializers/merge_request_serializer.rb

app/services/notes/create_service.rb

app/views/shared/issuable/_form.html.haml