Speedup User dashboard queries

The old queries were very slow in a puzzling case: where they matched
0 Action objects.  I'm not really sure why that is, but using extra()
makes for more straightforward SQL and the queries perform better.
Use select_related() on the user dashboard page

This reduces the number of queries substantially.  The difference was
about 3-4 queries per Action item when I was testing.
Merge pull request #1799 from pculture/speedup-user-dashboard

Speedup user dashboard