R1:ce953ea44790

Explicitly mark MFA challenges as "answered" and "completed"

Summary:
Depends on D19893. Ref T13222. See PHI873. A challenge is "answered" if you provide a valid response. A challenge is "completed" if we let you through the MFA check and do whatever actual action the check is protecting.

If you only have one MFA factor, challenges will be "completed" immediately after they are "answered". However, if you have two or more factors, it's possible to "answer" one or more prompts, but fewer than all of the…