Fix differential.find and the web API for the conduit console

Summary: differential.find was fataling in the console because of a bad
constant, and you couldn't submit requests requiring authentication because
the auth stuff didn't know to look for web sessions. Resolve these issues.

Test Plan: Hit differential.find successfully, ran a user.find query from
the web console without issues.

Reviewers: tomo

CC:

Differential Revision: 31