Review Board 4.0 introduced a new consistent way of setting up page

models and views in JavaScript, which coordinates the initialization and

events of page elements. This change moves all the datagrid pages over

to the new method.
To do this, the base datagrid template now fills in the page template

blocks to specify the model/view classes that the datagrid wants, along

with the attributes and options for those classes. This makes use of new

methods in DataGridJSMixin, which subclasses can override to provide

additional information.

Tested that all the datagrid pages were still working as expected in mobile

and desktop modes.
Unit tests pass.