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

spec/controllers/admin/instances_controller_spec.rb

@@ -0,0 +1,15 @@
+require 'rails_helper'
+
+RSpec.describe Admin::InstancesController, type: :controller do
+  before do
+    sign_in Fabricate(:user, admin: true), scope: :user
+  end
+
+  describe 'GET #index' do
+    it 'returns http success' do
+      get :index
+
+      expect(response).to have_http_status(:success)
+    end
+  end
+end