[Radiant] [Patch(?)] Loading of extensions' Page subclasses

[aslak hellesoy]

I'm working on an extension that defines HappeningPage < Page.

At the time where page.rb does a Page.load_subclasses (at the bottom),
none of the subclasses are loaded yet. To me it looks like
#load_subclasses scans radiant for _page.rb files and constantize the
classes, but it doesn't do the same for extensions' page subclasses.
As a result, extensions' subclasses get marked as #missing? => false

The attached patch fixes this. I'm not 100% sure if I'm right in
assuming this is a bug though. If someone can confirm, I'll stick it
in Trac.

Aslak