Update users ES index directly on save

(don't merge yet)
@czue @dannyroberts 
Waiting until tomorrow to test this out on staging, but locally it works (I see updates to users in ES even with pillowtop off).  Depends on https://github.com/dimagi/pillowtop/pull/91
maybe?  I also meant to move that import to the top before PRing.  I haven't really used django Signals before, could you give me a rundown some time?