We had a Submitter column used for review requests and reviews, which

rendered the username and avatars. We didn't have an equivalent for the

Users page, which would have had to duplicate a lot of the logic.
Now there's a single UsernameColumn, which displays a consistent

representation of the username and avatar. It takes the label and a

relation between the object and the user, and uses that relation to look

up the user and to augment the queryset.
The datagrids no longer use subclasses. Instead, they simply pass the

desired label and relation to UsernameColumn.
If new datagrids involving users are introduced in the future, they

should be able to use this column directly without needing to construct

another class.

Tested each datagrid, checking that the labels, usernames, and avatars

are all correct. Also checked that there wasn't a regression in query

counts from this change.