Simplify tests requiring admin login

We were repeating the same code over and over (with a few variants) to
setup tests which require an administrator. We can use a tag and
simplify the code.

spec/system/admin/dashboard/actions_spec.rb

@@ -1,12 +1,6 @@
 require "rails_helper"
 
-describe "Admin dashboard actions" do
-  let(:admin) { create :administrator }
-
-  before do
-    login_as(admin.user)
-  end
-
+describe "Admin dashboard actions", :admin do
   it_behaves_like "nested documentable",
                   "administrator",
                   "dashboard_action",