Refactor subscription functionality

Subscription functionality was hacked in as an extension which piggybacks on the notifications. This feature works great for what it is, but it adds too much noise to your notifications/mentions and you can't see things that were directed at you.

Propose the following changes:

1. Subscription notifications should be stored in a separate table from normal notifications
2. Create a new timeline to view them / MastoAPI extension to notifications to view them
3. After marking read they should be cleared. I see little value in keeping this history. If you want to see the history... go look at the user's profile and see their statuses.
4. We'll build a way to flip between notifications and subscriptions in FE

Users of clients that don't support this feature will still get push notifications for these messages. I don't think we can fix that because it's just the nature of diverse clients in this open ecosystem. Such is life.