Refactor MergeWhenBuildSucceedsService and incorporate feedback

CHANGELOG

app/controllers/projects/merge_requests_controller.rb

app/models/merge_request.rb

...vices/merge_requests/merge_when_build_succeeds_service.rb

app/services/system_note_service.rb

...ews/projects/merge_requests/widget/open/_accept.html.haml

...requests/widget/open/_merge_when_build_succeeds.html.haml

config/routes.rb

db/schema.rb

doc/api/merge_requests.md

lib/api/merge_requests.rb