Remove uid from prediction queries until personalization is supported

[dcrankshaw]

Currently, users must supply a uid as part of the prediction request body but it is never used and must always be 0. We should remove the uid field entirely. The simplest way to do this is to just stop checking for uid when we parse the prediction request JSON and hardcode the uid as 0.

Replace this line with

long uid = 0;

and update the corresponding schema here