Provide access to Phriction via Conduit

Summary:
Provide three Phriction methods for programmatic access to Phriction:

• phriction.info: get information about a document
• phriction.history: get change history for a document
• phriction.edit: create or update a document

I moved all the editing/creation logic into a new PhrictionDocumentEditor to
share code between the Conduit and Web edit pathways.

Test Plan: Got info and history via conduit. Edited and created new pages via
conduit and web.

Reviewers: hsb, jungejason, tuomaspelkonen, aran, hunterbridges

Reviewed By: hunterbridges

CC: skrul, aran, hunterbridges

Differential Revision: 866