Review reminders: migration

How do we want to perform the migration from the old notification system to the new review reminders? How fast should the switch-over be? Do we want it to be opt-in / simply increase the visibility of the feature flag for a few weeks? Do we just unilaterally migrate everyone from the old system the the new one in a single update, and ensure that we test it properly in alpha and beta?

We should create some migration code that detects if the old notification settings are on (via `PreferenceUpgradeService`). For example, if "Notify when: more than 10 cards due" is selected, then a review reminder for all decks with a card trigger threshold of 10 should be created when the app is loaded up with the new update for the first time. We can ignore migrating the old `vibrate` and `blink light` settings, as we've decided to let the OS handle those.

Finally, we should delete the old legacy notifications code (anything annotated with `LegacyNotifications`, `NotificationFragment`, etc.).