Always require MFA to edit contact numbers

Summary:
Depends on D20023. Ref T13222. Although I think this isn't strictly necessary from a pure security perspective (since you can't modify the primary number while you have MFA SMS), it seems like a generally good idea.

This adds a slightly new MFA mode, where we want MFA if it's available but don't strictly require it.

Test Plan: Disabled, enabled, primaried, unprimaried, and edited contact numbers. With MFA enabled, got prompted for MFA. With no MFA, no prompts.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D20024