The column registration had a nasty bug when used with subclasses of

subclasses of DataGrid.
If a subclass was being used as a normal datagrid, rendered on a page,

it would populate its registry of columns. If a subclass of that was

then used, it would inherit the registry, ignoring any custom columns

that it may itself provide.
Instead of storing the column registry on the datagrid itself, we now

have a map of classes to columns. We can then reference those registries

by the specific class we care about, instead of inheriting registry from

parent classes.

Loaded the All Review Requests page on Review Board, and then loaded the

Dashboard. Both had their own lists of available columns, instead of the Dashboard

inheriting the one on the All Review Request page.
Unit tests pass.