Add a flag to the current user's session if they went through a MFA verification

[MarkusH]

This is a feature request that stems from #60.

Feature request

As a developer, I want to know if users went through an MFA process and possibly restrict access to a view if they haven't. For that, it'd be useful to store a flag, like kagi_verified=True in the current session.

Since I also use authentication through other means, such as OIDC or SSO in general, I want to be able to mark those users as "verified" as well. For that, I'd be good to have a utility function to do so.