Move back to using fpm, after passing in all the needed params.

It turns out, in my experiments, that only SCRIPT_FILENAME was really
needed (I must have accidentally deleted it at some point).  But I
added in a bunch of other params as well, as specified at
   https://secure.phabricator.com/book/phabricator/article/configuration_guide/

Maybe these will help things.

Auditors: alpert

Test plan:
Fingers crossed.  Will install on phabricator, restart nginx, and
visit https://phabricator.khanacademy.org/D1234