R1:fec5ea8f9dac

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

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