Rename BaseController so we can deprecate it.
Move Admin pages to using LoggedInHandler.

This does a few things:
- Move to using LoggedInHandler so we have a consistent way of handling current user status in Admin Controllers.
- Changes how we populate template_values for Admin Controllers so we can always inject a user_bundle dict for templates to reference the current user
- Uses in-code authentication checks, instead of the GAE requires_admin permissions thing, but I haven't removed that yet