Put gem templates through parent's app controller

* The style guide template should render through the parent app’s
application controller and template — eg showing the parent’s
application name, main navigation and so on.
* Following guide from
http://guides.rubyonrails.org/engines.html#using-a-controller-provided-b
y-the-application
* Prior to this the style guide route was not working in app’s using
the gem because GovukAdminTemplate::ApplicationController wasn’t
available