Allows MR authors to have the source branch removed when merging the MR

CHANGELOG

app/controllers/projects/merge_requests_controller.rb

app/models/merge_request.rb

app/services/merge_requests/create_service.rb

app/services/merge_requests/merge_service.rb

app/services/merge_requests/update_service.rb

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

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

...rojects/merge_requests/widget/open/_not_allowed.html.haml

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

spec/models/merge_request_spec.rb

spec/services/merge_requests/create_service_spec.rb

spec/services/merge_requests/merge_service_spec.rb

spec/services/merge_requests/update_service_spec.rb