Updated to set the page layout when rendering custom actions

Did it the same as ResourceController with an ugly cut and paste, but moving determine_active_admin_layout to BaseController seemed uglier without bigger changes and was hurting my Java brain waaay to much.

I didn't add any tests as there were none in the original as well as the fact that heaps of tests were failing for me in the build.  

I did test that it worked in my local active admin project.