Clean up bits of the datagrid code.

Every time someone submits a contribution to the dashboard or other
datagrids, they copy our old column code, which doesn't use super() for
parent constructors, and doesn't format docstrings correctly.

This change fixes up the datagrid code to be more consistent with newer
code going into Review Board, and to also clean up our documentation a
bit. There are no functionality changes.

Enabled every column. No problems.