Clean up legalpad sign UI

Summary: Moves stuff to ObjectBox, Error UI

Test Plan: Signed Document, Missed a Form Field, Resigned Form Document. Fixes T4037

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7463