Fix get_url_params_except and augment_queryset.

My get_url_params_except method forgot to return the value, which meant that
we'd often see "None" in the URL parameters. I've also changed things a bit to
avoid adding empty query parameters when this method returns nothing.

This also fixes the datagrids code to always call Columns' augment_queryset()
methods. By adding a query string like ?sort=people (which is a non-sortable
column), one could drop the datagrid through a code path where it wouldn't call
augment_queryset, leading to bizarre error messages like this:

    'ReviewRequest' object has no attribute 'draft_summary'

This change fixes that up, fixes the obsolete docstring for
post_process_queryset.

Testing done:
Played with a bunch of different sort parameters on various datagrids,
including adding query strings that included non-sortable columns.

Fixes bug 2565.

Reviewed at https://reviews.reviewboard.org/r/5373/