Add setting to only allow merge requests to be merged when all discussions are resolved

app/controllers/projects_controller.rb

app/models/merge_request.rb

app/views/projects/_merge_request_settings.html.haml

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

...ge_requests/widget/open/_unresolved_discussions.html.haml

...ased/20968-add-setting-to-check-unresolved-discussion.yml

...31004_only_allow_merge_if_all_discussions_are_resolved.rb

db/schema.rb

doc/api/projects.md

lib/api/entities.rb

lib/api/projects.rb

spec/controllers/projects/merge_requests_controller_spec.rb

spec/factories/merge_requests.rb

...equests/check_if_mergeable_with_unresolved_discussions.rb

spec/models/merge_request_spec.rb

spec/requests/api/projects_spec.rb