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