The introduction of avatar support caused performance degradations in

the datagrids due to the way profiles are fetched. Every entry had a

get_profile() call, which performed a database lookup, which made the

dashboard and other datagrids very slow.
To solve this, the columns now do a select_related() on the profile

field, and get_profile() has been updated to reuse the same caching

variable that's populated for the relation. This means that if a profile

is ever included using select_related() or looked up using the field

relation, we can call get_profile() without performing an additional

lookup.

Verified that the dashboard, All Review Requests, and user pages no

longer have a profile query per datagrid entry.
Unit tests pass.