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/views/admin/instances/index.html.haml

@@ -19,9 +19,8 @@
 - unless whitelist_mode?
   = form_tag admin_instances_url, method: 'GET', class: 'simple_form' do
     .fields-group
-      - Admin::FilterHelper::INSTANCES_FILTERS.each do |key|
-        - if params[key].present?
-          = hidden_field_tag key, params[key]
+      - InstanceFilter::KEYS.each do |key|
+        = hidden_field_tag key, params[key] if params[key].present?
 
       - %i(by_domain).each do |key|
         .input.string.optional