Allow Herald to "Require legal signatures" for reviews

Summary:
Ref T3116. Add a Herald action "Require legal signatures" which requires revision authors to accept legal agreements before their revisions can be accepted.

  - Herald will check which documents the author has signed, and trigger a "you have to sign X, Y, Z" for other documents.
  - If the author has already signed everything, we don't spam the revision -- basically, this only triggers when signatures are missing.
  - The UI will show which documents must be signed and warn that the revision can't be accepted until they're completed.
  - Users aren't allowed to "Accept" the revision until documents are cleared.

Fixes T1157. The original install making the request (Hive) no longer uses Phabricator, and this satisfies our requirements.

Test Plan:
  - Added a Herald rule.
  - Created a revision, saw the rule trigger.
  - Viewed as author and non-author, saw field UI (generic for non-author, specific for author), transaction UI, and accept-warning UI.
  - Tried to accept revision.
  - Signed document, saw UI update. Note that signatures don't currently //push// an update to the revision, but could eventually (like blocking tasks work).
  - Accepted revision.
  - Created another revision, saw rules not add the document (since it's already signed, this is the "no spam" case).

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: asherkin, epriestley

Maniphest Tasks: T1157, T3116

Differential Revision: https://secure.phabricator.com/D9771