Improve notifications architecture

[Skud]

Forked off from #596.

Notifications need to be rearchitected. See proposed improvements to notifications on Growstuff Talk for the overall design etc.

My work in progress is on this branch: https://github.com/Skud/growstuff/tree/follow_notification

[cesy]

It would be great if someone could pick this up soon. Skud won't be working on it for a while, but her work is all public.