This review request creates RB.Admin.PageView, a new base page view that is 

specific to admin views. Existing views in reviewboard/static/rb/js/admin/views 

that don't extend from a specific view are updated to extend RB.Admin.PageView. 

They also call the parent view's initialize and renderPage as needed. This 

change will help with the implementation of the admin setup banner, which needs

to be displayed on all admin pages.

Existing unit tests all pass and a manual inspection of admin pages shows that

pages still render correctly.