Allow users to apply group labels on Issues/MRs

app/controllers/dashboard/labels_controller.rb

app/controllers/projects/issues_controller.rb

app/controllers/projects/labels_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/finders/issuable_finder.rb

app/models/label.rb

app/services/issuable_base_service.rb

app/services/projects/autocomplete_service.rb

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

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

spec/services/issues/create_service_spec.rb