Legalpad - add signature page

Summary: Fixes T3481. Sort of - this thing be very ugly. Also it assumes that you'll "always" want to sign terms. I was thinking in a future diff that should be optional as well as configurable, though it was unclear to me if either was worth pursuing...  Generally very hideous as the three main elements (PHUIDocument, AphrontErrorView, and AphrontForm with an AphrontFormInset) have never really played together before.

Test Plan: agreed to some test terms. noted UI displayed nicely. reloaded and noted UI told me I had signed it already. Went to different terms and filled them out wrong and got sensical errors.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3481

Differential Revision: https://secure.phabricator.com/D6399