Email subscription overview on profile page

Users should see the subjects and groups they're subscribed to on their profile page and be able to unsubscribe from any of them.
This should probably be a modal with a bunch of checkboxes.
The button to synchronize subjects from AuthSCH should also be here, if the backend part (https://github.com/kir-dev/konzisite-api/issues/143) is ready

Start at the profile page entry point and review the linked backend issue #143 to confirm whether the required synchronization support is ready. Map the subscribed subjects and groups, unsubscribe controls, and AuthSCH synchronization into the profile UI; the work is done when users can view and manage subscriptions there.

Written by the indexing model from the issue text.