This updates RB.PageView to simplify some common operations that are

required by subclasses.
An isPageRendered flag is now set after a page has been rendered.

Callbacks can use this to ensure that they're working with a page that

is now fully rendered to the DOM.
The main sidebar content area is now accessible through a $mainSidebar

attribute. This prevents pages from having to look up the correct

element themselves.
Pages can now override onMobileModeChanged() to provide custom logic

when switching between desktop/mobile mode, instead of having to set up

a custom signal handler (which could easily and accidentally conflict

with the base page's _onMobileModeChanged signal handler).

Made use of all these features in an upcoming change.
Unit tests pass.