R1:c7718d3a2126

Ask users to sign Legalpad documents before requiring they enroll in MFA

Summary:
Depends on D18789. Ref T13024. See PHI223. Currently, if `security.require-multi-factor-auth` and Legalpad "Signature Required" documents are //both// set, it's not possible to survive account registration, since MFA is requiried to sign and signatures are required to add MFA.

Instead, check for signatures before requiring MFA enrollment. This makes logical sense, since it's silly to add MFA if you don't agree to a Terms of…