Abstract away stars – support generic "actions" with associated counters

[rrbrambley]

It would be great if instead of having a star count on Posts, you could have any number of generic actions (e.g., "lol", "downvote," "num_views," etc.) – all of which would have an associated counter, just like with the current starring feature on Posts. Issue #354 is a request for starring on Messages, and so maybe the best way to implement this feature would just be to forget about adding starring to messages, and instead support generic actions.

These actions could be defined by the Channel. When creating a Channel, I would specify a set of actions that could be applied to any message in that channel. A user who can write to that channel could then invoke the action (much like starring) on any message that he or she sees. This would enable all sorts of new types of apps to exist. Some use cases:

1. Upvotes and downvotes. This would allow an app to implement a ranking algorithm (e.g. hacker news).
2. Reactions/Emotions, enabling an app like Buzzfeed: lol, win, fail, omg, wtf, etc.
3. Number of views/reads. A client could mark something as viewed/read. This could be used for traditional ephemeral messaging, or an app that would only allow the first X many readers to see a message.