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
