We previously introduced a prefetch_related() optimization for list

resources, helping to get the query counts down for any referenced

ManyToManyFields.
We also had a select_related() call, but this wasn't good enough.

Calling select_related() without parameters will only operate on

ForeignKeys that don't allow for NULL values.
Now, we build a list of fields to explicitly pass to select_related().

This has the benefit of working with fields that allow NULL values, and

prevents looking up fields we're not going to return in the payload.

Primarily tested with /api/review-requests/. When spanning pages, we

had a query count of 27 prior to this change (factoring in only the

queries that were part of the API payload, and not ones like the session

or siteconfig).
After this change, that was down to 6 (1 for the main query, 1 for total

numbers of items, 4 for the relations).