MFA step-up auth

[porcellus]

🚀 Feature

We want to allow apps to have different MFA requirements in different parts of their apps (e.g.: add a higher requirement for the security settings page)

This will be solved separately from the base MFA case (#554).

Possible issues

• We need to allow requiring factors based on time
• We need to solve/fix the back-button issue (if SessionAuth redirects you to an MFA screen, the back button points to the screen that redirected you)
• We need to solve/fix the issue where the screens SessionAuth redirects you (emailverification and MFA screens) do not redirect you back after you're done with them.

Solutions to explore

• Instead of redirection, SessionAuth could show the factor/ev screens (e.g.: in a modal)