R1:add7bc418dcb

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…