Horizon example: notifications

[deontologician]

A simple notification scheme:

Users create documents in the 'notifications' collection.
Each document has a sender, receiver and read field
Permissions would be:

• collection('notifications').store({sender: context.id, recipient: any(), read: false})
• collection('notifications').findAll({ recipient: context.id }).anyRead()
• collection('notifications').update({ recipient: context.id, read: any() })

Users create a changefeed like hz('notifications').findAll({recipient: userId}).watch(), and can do store to send a notification to another user, and update to mark their own notifications as read.