User notifications

[ghost]

At the moment there is no User notifications feature (a la facebook) for Hasjob.

User notifications feature is to solve scenarios like: How do you communicate to a logged-in user that his jobpost failed to be published because the email verification bounced? (see #226)

Ideally we want this feature to be available to all our Flask apps by being implemented as an API in lastuser (see issue #114) but each of our apps need our Notifications class to behave differently depending on the context of the app. So, at this point, implementing it locally in HasJob and then observing bits that are viable to have extrapolated into Lastuser makes the feature easier to design the feature itself.

From the referenced issue and w.r.t to Hasjob, our Notifications feature should ideally support the following:

1. Multi-channel notifications
   • Try notifying user through E-mail, if not then in-app, if not then SMS.
2. Custom user set notifications
3. Respond to notification from within the notifications inbox (a la Gmail app in Android - "Archive" "Reply" right from the notifications slider screen)

In a perfect world, we'd also like to have the ability to delay notifications and withdraw notifications but this is tied to the design problem of Universal undo across our apps. How do we rollback in-app actions?