Allow applications to call Conduit directly

Summary:
Sorry this took so long, had a bunch of stuff going on today.

Separate the actual core part of making conduit calls from the controller, so the application can make conduit calls without needing to invoke HTTP or redo auth. Generally, this lets us build more parts of the application on top of Conduit, as appropriate.

This diff can be simplified, but I wanted to unblock you guys first. I'll followup with a cleanup patch once I have a chance.

Test Plan: Ran unit tests, ran calls from the conduit API console, and ran calls over arc.

Reviewers: nodren, 20after4, btrahan, vrana

Reviewed By: 20after4

CC: aran, svemir

Maniphest Tasks: T945

Differential Revision: https://secure.phabricator.com/D2718