Legalpad - a few rough edges smoothed...

Summary:
does a few smallish things... Ref T3116

 - adds an action to "sign document", thus improving visiblity of this feature from 0 to some value more than 0
 - adds a crumb on the edit page to get back to the view page
 - warns the user on the edit page IFF signatures exist for the current version that their edits could invalidate those signatures
   - adds a "needSignatures" option to the Document Query class

Test Plan: click the new UI elements and they worked. edited a document with signatures, noted warning UI, edited anyway, noted warning UI correctly disappeared on new edit. also verified a single signature had the correct translation

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D7983