Fix flashes partial render error for controllers which don't inherit from application controller (#2400)

* Add failing spec for oauth/authorized_applications controller

* Use explicit reference to flashes partial from admin layout

Because some of the controllers which use the admin layout do not inherit from
application controller, this partial is not in their view path.

app/views/layouts/admin.html.haml

@@ -13,7 +13,7 @@
       .content
         %h2= yield :page_title
 
-        = render 'flashes'
+        = render 'application/flashes'
 
         = yield
 

spec/controllers/oauth/authorized_applications_controller_spec.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Oauth::AuthorizedApplicationsController do
+  render_views
+
+  before do
+    sign_in Fabricate(:user), scope: :user
+  end
+
+  describe 'GET #index' do
+    before do
+      get :index
+    end
+
+    it 'returns http success' do
+      expect(response).to have_http_status(:success)
+    end
+  end
+end