Login with passkey design revamp due to storage partitioning

[esaminu]

Description

Since storage partitioning was added to chrome and exists in Safari, we are no longer able to access localStorage from an iframe and can therefore no longer access the webauthn_username value that was used to keep track of the user logged in to the signer app.

To fix this #207 added a passkey prompt that gets triggered when the user focuses on the email address input. If the user selects a passkey when they are prompted, the login flow will resume with that key, otherwise the user can dismiss the prompt and enter an email address in order to be redirected to the verify email page.

This ticket will track the design decision making on how we should manage the option of passkey sign in vs sign in with email given the storage partitioning changes.