R1:961fd7e8491e

In Legalpad, prompt for MFA at the end of the workflow instead of the beginning

Summary: Depends on D19895. Ref T13222. This is a simple behavioral improvement for the current MFA implementation in Legalpad: don't MFA the user and //then// realize that they forgot to actually check the box.

Test Plan:
- Submitted form without the box checked, got an error saying "check the box" instead of MFA.
- Submitted the form with the box checked, got an MFA prompt.
- Passed the MFA gate, got a signed form.