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 privileges to change the value.

Test Plan:
Edited Legalpad documents as an administrator and non-administrator before and after the change, with and without signatures being required.

Couldn't reproduce the original issue, but this version is generally more correct/robust.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20311