Teach PositionQuerySet.currently_active to understand past=True

Fixes #1455
Ensure that currently_active understands blank start and end dates
Add a raw_query_with_prefetch function for faking select_related

This is useful to cut down the number of queries required when you'd
like to do a select_related on a RawQuerySet and optionally
prefetch_related on those fields in turn.
ZA: Fix the organisation party sub-page display of people

Previously this produced buggy and confusing results due to, for
example, not taking to account that people might change party between
terms of office, and various other complications.

Fixes #1450
Merge remote-tracking branch 'origin/1450-fix-organisation-party-pages'