R1:8dca5cdb5a04

Add Conduit method to disable user

Summary: There's no method for enabling users somewhat intentionally.

Test Plan: Disable myself (oops, this is probably my last diff ever).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2497