API: Add more fields to the high performance CSV queryset

[chigby]

Right now, only a subset of incident fields are compatible with the higher performance queryset for the CSV API. These are the fields that we have written some custom Django ORM code to query the values from the DB directly, rather than computing them per-row in Python. We would like to add as many fields as possible to this group, so that all possible API queries can be returned using the faster code.

Follow up for #1650