Add a base models/views for pages.
Review Request #9151 - Created Aug. 25, 2017 and submitted
RB.PageView, which are base classes used
for any pages managed by
RB.PageManager. These currently don't have
any real functionality, but will allow us to move some logic further
down in the future.
Unit tests passed.
Tested all types of pages (datagrid, review request, diff viewer,
review UIs) to make sure everything worked.