Implement Notifications

[kentonv]

Keeping track of activity in Sandstorm seems to be a pain point. We need a way for grains to notify users of:

• Activity: Something in this grain has changed, you might want to review it.
• Attention requests: Something in this app requests your specific attention (e.g. someone tagged you).

I think it would make sense to implement something in Sandstorm that roughly emulates Rocket.Chat's internal notifications model. In Rocket.Chat, a channel in the sidebar is gray if you've read everything, white if there is new activity; we can do similarly in the Sandstorm sidebar and grain list. In Rocket.Chat, tagging a person causes them to be actively notified through a desktop notification and possibly email. Additionally, channels in the sidebar are annotated with numbers indicating the number of unread tags in that channel. In Sandstorm, we could do similar, and also employ the bell menu as another delivery mechanism.

I've written up some design notes here:

https://oasis.sandstorm.io/shared/X_xCdatT9u8nTWjDfMC4WdHS0JOlLLKb1yus4zaiFha

One implemented we'll prioritize updating Rocket.Chat, Wekan, and Etherpad to use the system.

@wtogami does this seem headed in the right direction?

[ocdtrekkie]

This is one of my long-time wish list items! :D