Allow session cookie name to be customized

This was manually cherry-picked out out edx:master.


@jbau I forgot I had this commit sitting around. I _did_ include this commit with my staging deploys on Friday.

I originally made this change, thinking this had something to do with why my changes weren't taking effect earlier. It turned out to be because I was improperly defining `SESSION_COOKIE_NAME` at the top-level of secrets.

So:
- Do we still want/need this?
- I'm _guessing_ we still want this for "sensible defaults"?
- Should `SESSION_COOKIE_DOMAIN` default to empty string or something else?