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.
• Tried to sign another form, hit MFA timed lockout.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19896