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.

• 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).