fix: users should return to previous screen on mfa required

[Benehiko]

This PR adds the current requested page as the return_to page when redirecting to login with aal=aal2.

This is especially true for when we want to redirect back to the original page the user was navigating to. This fixes the problem where a user has logged in and navigates directly to the settings page but still needs to complete MFA.
After completing the login flow the user is now stuck on the /welcome page.

Related Issue or Design Document

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• I have referenced an issue containing the design document if my change introduces a new feature.
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• I have added tests that prove my fix is effective or that my feature works.
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