Allow users to access some settings at the "Add MFA" account setup roadblock

Summary:
Depends on D20006. Ref T13222. Currently, the "MFA Is Required" gate doesn't let you do anything else, but you'll need to be able to access "Contact Numbers" if an install provides SMS MFA.

Tweak this UI to give users limited access to settings, so they can set up contact numbers and change their language.

(This is a little bit fiddly, and I'm doing it early on partly so it can get more testing as these changes move forward.)

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D20008