Add read attribute to notification resource

[mcbouslog]

A read attribute on the notification resource would be helpful, with a boolean to denote read or unread status.

PFE is currently using the delivered attribute on the notification resource for the header notifications count and UI on the notifications page for read or unread status of notifications. This can be confusing when the Talk API (properly, as intended) toggles delivered from false to true when a user is emailed according to their notification email settings.

Please see this comment from related PFE Issue 4727 for additional information.

Notable code:

• https://github.com/zooniverse/Talk-Api/blob/master/app/controllers/notifications_controller.rb#L8
• https://github.com/zooniverse/Talk-Api/blob/master/app/mailers/notification_mailer.rb#L9
• https://github.com/zooniverse/Talk-Api/blob/master/app/mailers/notification_mailer.rb#L57-L59

[eatyourgreens]

I came across this again this morning. I asked a question on Every Name Counts yesterday. Two volunteers replied overnight but their replies were marked as already read when I checked my notifications on the site today.