Ask for notification permission when enabling account notifications

Currently the app is only asking for the notification permission during onboarding. But if the user doesn't grant the permission then, the app should ask again when the user enables notifications for an account (either when adding an additional account or via account settings).

We'll also have to handle the case where the user has denied the permission already and point them to the correct Android screen to reverse that decision.

Open question: What should the app do when it notices the notification permission has been revoked, but notifications are enabled inside the app?