Possible fix for previewing pages with inherited templates.

There is an issue with previewing pages, which uses templates inheritance.
Rails fails to find partials from views/refinery/pages and views/application folders, because `Refinery::Pages::Admin::PreviewController` not inherited from `Refinery::PagesController`.
It fails with exception message: 
```
Missing partial refinery/pages/admin/preview/inherited_partial, refinery/admin/inherited_partial with {:locale=>[:en], :formats=>[:html], :handlers=>[:erb, :builder, :coffee]}.
```
Example app for this issue:
https://github.com/Rakoth/refienry-2.1.0-pages-preview