Pholio - robustify submission errors to retain image edits

Summary: we should build all the image stuff on every post and use that posted image data if there's an error. this diff makes that so. Fixes T4380.

Test Plan: made a mock with no title, tried to save it, and was delighted to see my images still there. edited a mock - removing the title and adding images - verified edits showed up after erroneous submission. added a title and submitted and changes were saved.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4380

Differential Revision: https://secure.phabricator.com/D8197