Used Django's builtin redis cache backend

[bmispelon]

Fix for #1023

Two big questions that I don't have the answer to (yet?):

• [ ] Should we re-populate the cache (somehow transferring the data from memcached to redis) as we deploy the default cache? How big of an impact would it have to clear the site's cache all at once?
• [ ] Will the docs-pages cache change seamlessly work as we change the backend (I think yes, but I haven't tested)
• [ ] Should we consider using hiredis to improve performance? I believe all that would be needed is to use redis[hiredis] in the requirements.