Datagrid column sorting relied on the db_field attribute of a Column,

but if a column needed to dynamically compute which field to use, it was

pretty much out of luck without resorting to hacks.
Instead, we now use get_sort_field(), which returns the field used for

sorting. This defaults to db_field, but can be overridden by subclasses.

Tested sorting of standard fields.
Used this for the review request ID field fix in Review Board, which

suddenly was able to be sorted correctly, both on global sites and

Local Sites.