Add the ability to star a conversation

[matthewpalmer]

This won't be ready for a while.

Need to figure out precisely what a star means:

• appears at the top of the threads list?
• receive push notifications?
• adds visual identifier next to message?
• more like a "read later" thing? can you view a list of all starred convos? do we need user accounts?
• do we have star counts for all convos? can people order threads by number of stars?

[matthewpalmer]

• If we do this, it'd be cool to have a star count across all users (or maybe a +1 / -1 thing?). This would then require changes to the backend, so I'm holding off doing this for a little while