User.get_profile() attempts to cache or make use of a cached result

from a previous call. It used to make use of _profile_set_cache, which

was the name used in Django 1.6 and 1.11. Newer versions use different

mechanisms for cache, for both prefetch_related() and

select_related(). This was a regression from Review Board 4.
This change fixes that support and expands upon it. A new series of unit

tests have been added that ensure caching works exactly as expected in a

variety of situations.
As part of this, various datagrid unit tests have been updated to once

again have reduced query counts. These counts were initially increased

in commit ceab149f415ade7432a3ff6e625beb0cdc5645f2, thought to have been

due to middleware changes, but were actually due to the Django relation

caching changes.

Unit tests pass.