Backend Support for Notification Settings Redesign

[AdrianAndersen]

This PR add support for the redesigned notification settings.

New Features

• Scheduled web push notifications, I set up a new table with scheduled web push notifications that a cron job periodically checks to see if there are any due notifications. If a notification is due it sends it.
  • Scheduled notifications are regenerated when a user modifies their preferences, in addition to being scheduled/cancelled when a user books/unbooks a classe
• A completely redesigned frontend, see PR in reservo-web
• Stored Push Subscriptions now include notification grants, for booking, community and reminders
• For slack reminders, we store a separate variable slack_reminders in the user preferences

One important note is that users will have to re-enable class reminders, since we now evaluate whether they want them or not with another variable. We could write a migration for it, but I think it is such a simple thing to do either manually in the DB or just each user for their own. Let me know what you think.

This PR depends on mathiazom/rezervo-web#160

Resolves #50