Add domain search/filter to the "Federation" (/admin/instances) page (#10071)

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

@@ -11,6 +11,20 @@
   %div{ style: 'flex: 1 1 auto; text-align: right' }
     = link_to t('admin.domain_blocks.add_new'), new_admin_domain_block_path, class: 'button'
 
+= 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]
+
+    - %i(by_domain).each do |key|
+      .input.string.optional
+        = text_field_tag key, params[key], class: 'string optional', placeholder: I18n.t("admin.instances.#{key}")
+
+    .actions
+      %button= t('admin.accounts.search')
+      = link_to t('admin.accounts.reset'), admin_custom_emojis_path, class: 'button negative'
+
 %hr.spacer/
 
 - @instances.each do |instance|