R1:930cc7a6dd70

Generalize Legalpad validation logic for "Require Signature"

Summary:
See downstream <https://phabricator.wikimedia.org/T208254>.

I can't actually reproduce any issue here (we only show this field when creating a document, and only if the viewer is an administrator), so maybe this relied on some changes or was originally reported against older code.

Regardless, the validation isn't quite right: it requires administrator privileges to apply this transaction at all, but should only require administrator…