Add an app for managing multi-"page" config forms.

This makes it easy for a view to provide collections of configuration
pages with a sidebar used for navigation. Each page can have one or more
forms, each of which can be submitted individually.

This is a generalization of the My Account page from Review Board 2.0,
and the Team Admin page from RBCommons. All the JavaScript and CSS were
directly copied from Review Board (with the addition of the two macros
the .less file needs).