Allow or not merge MR with failed build

CHANGELOG

app/assets/javascripts/project_new.js.coffee

app/controllers/projects_controller.rb

app/models/merge_request.rb

app/views/projects/_merge_request_settings.html.haml

app/views/projects/edit.html.haml

app/views/projects/merge_requests/widget/_open.html.haml

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

...ojects/merge_requests/widget/open/_build_failed.html.haml

...101_add_only_allow_merge_if_build_succeeds_to_projects.rb

db/schema.rb

lib/api/merge_requests.rb

...ures/merge_requests/only_allow_merge_if_build_succeeds.rb

spec/models/merge_request_spec.rb

spec/requests/api/merge_requests_spec.rb