Phriction - add "create" conduit endpoint
Summary: Fixes T6262. Ref T4029. Also gets us ready for T5873 for these end points. I can file something new about someday adding phriction.query, etc but I think we'll remember and can look at that post T5873.
Test Plan: made a document via conduit and it worked!
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D10813