Fix bug 1008431: Do not prefetch banners on user profile page.

For some reason I don't fully understand, prefetching banners related to
banner variations for a user's links causes some banners to not be 
fetched and raise DoesNotExist instead. I imagine it has something to do
with banner variations being a GenericRelation.

This adds some queries to the profile page, but they're pretty cacheable
by django-cache-machine.