Various fixes and improvements (#12878)

* Fix unused role routes being generated

* Remove unused JavaScript code

* Refactor filters code to be DRYer

* Fix `.count == 0` comparisons to `.empty?` in views

* Fix filters in views

app/controllers/admin/instances_controller.rb

@@ -62,7 +62,7 @@ module Admin
     end
 
     def filter_params
-      params.permit(:limited, :by_domain)
+      params.slice(*InstanceFilter::KEYS).permit(*InstanceFilter::KEYS)
     end
   end
 end