User notifications #19
Description
We need to store and follow user notification preferences. Broadly, this includes for which categories of notification they want emails, and - possibly - for which categories of notification they want push notifications.
This includes, then, notification categories, which have not yet been implemented.
Email notification preferences currently live in the PHP back-end; we will need to migrate that data.
(This is currently a low priority enhancement because the first planned production use of the notification-service is for new account signups, and by definition the user cannot have configured any preferences yet.)
Push notification preferences is currently an open question, which is why I'm starting this issue now; see #18. Should push notification preferences be account-wide or device-specific? If they are device-specific, and we use Firebase data messages, we can rely on the mobile app to filter accordingly; we'll still be sending the message, but apparently Firebase Cloud Messaging is a free service, so there is at least not a financial impact of sending extra messages. (And, in theory, the app could save a call to retrieve the notification list if it knows every notification is sent via a data message.)