Phame - move over post creation and edit to use an editor

Summary: This adds the bare minimum transaction set for editing posts. Fixes T7626 because now files will be correctly attached to phame posts on subsequent edits. Future work here should be adding transaction types like TYPE_BLOG for when posts are moved between blogs, TYPE_VISIBILITY for when posts are moved to published, etc. Nothing too tricky there but keeping this diff relatively small seems prudent.

Test Plan: made posts successfully. also made errors like no title, no phame title, and duplicate phame title and got correct errors. added a file to a post and verified file has phame post in "attached" tab, which should fix T7626.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7626

Differential Revision: https://secure.phabricator.com/D12864