Instances list in admin (#2095)

* Add admin/instances index action

* Add link to instances admin page

* View lists instances

* Instances, grouped by domain, ordered by count

* Use Account.remote scope

* Extract method: Account.by_domain_accounts

app/controllers/admin/instances_controller.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Admin
+  class InstancesController < BaseController
+    def index
+      @instances = ordered_instances.page(params[:page])
+    end
+
+    private
+
+    def ordered_instances
+      Account.remote.by_domain_accounts
+    end
+  end
+end