Switch tests to use PostgreSQL, same as main site.
Remove django-mptt, replace with native CTEs.

I have a nagging concern with django-mptt - having to do the rebuild()
in the test, for example - plus I worry about slow insertion as the
database grows, and general fragility/complexity. Also, wanting to
have a default insertion order display made displaying by title need
something different anyway.

So this is switching back to a simple adjacency list, using CTEs to do
recursive descendant/ancestors.
Improve display when dates provided, but not times.