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!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6262, T4029

Differential Revision: https://secure.phabricator.com/D10813