Crud._distinct_query should work in backends other than postgresql

[EliAndrewC]

Moved here from the (now private) original Sideboard issue tracker.

The SQLAlchemy
http://docs.sqlalchemy.org/en/rel_0_8/orm/query.html#sqlalchemy.orm.query.Query.distinct
method only works on specific columns with Postgresql. This means that there's a significant difference between the two forms of distinct that sqlalchemy supports:

>>> [tag.name for tag in session.query(Tag).all()]
[u'Male', u'Male', u'Ninja', u'Pirate']
>>> session.query(Tag).distinct(Tag.name).count()
4
>>> session.query(Tag.name).distinct().count()
3

This is probably fine as long as we document this; I'd generally recommend using postgres everywhere, but it would be nice to not have this restriction. This would require a small but non-trivial rewrite to how we're performing these queries, and we should un-skip the relevant tests.