Better caching of `SELECT upload_imageattachment.....`

According to:
https://rpm.newrelic.com/accounts/263620/applications/2779374/databases#id=279160018

we relatively spend a ton of time checking for image attachments on answers when probably 99.9% of answers don't have an attachment. django-cache-machine doesn't cache this query so we keep calling it over and over. I vaguely remember that it's because django-cache-machine punts on caching empty lists because it can't invalidate them. Or something.

Anyway, this should make things better. The branch is based on the DDT update branch so that you can see that the caching is actually working. 

r?