Investigate using Subscriptions/ActionCable to help with cache invalidation

[acoffman]

Right now we use refetchQueries and polling in most places to handle stale data in the apollo cache. The "proper" way to do this would be to push events to the clients asking them to refresh their caches. We'd need to come up with an example use case to prove out and then look into how to configure and deploy actioncable in production.