OSQA-824: Enable Django template caching

Django loads and parses templates for every request, so this significantly improves OSQA performance.

* I've checked over the custom template tags for thread safety and they look fine,
  so this change should be safe:
  https://docs.djangoproject.com/en/dev/howto/custom-template-tags/#template-tag-thread-safety
* Profiling indicated the other major contributor to CPU/request time was Django's lack of database
  connection pooling/persistent connections. That's best handled with an external daemon:
  http://meta.osqa.net/questions/11381/is-osqa-cpu-bound

git-svn-id: http://svn.osqa.net/svnroot/osqa/trunk@1238 0cfe37f9-358a-4d5e-be75-b63607b5c754