Fetch recent history on app open

[WesleyAC]

We should fetch some recent message history as soon as the user opens the app (right after we get a response from /register), rather than waiting until the user selects a topic/stream.

Probably we should fetch a couple hundred messages at least, not sure exactly what the right number is (perhaps the answer is "all of the messages", or maybe "all of the messages if the user is not on a metered network"). It might make sense to check the webapp code to see how that does this.

One of the main motivating issues here is for small orgs, if there are only a few dozen messages a day, we should immediately grab them. The exact behaviour on large orgs probably matters less, but we should consider fetching all unread PMs on load at least.

[gnprice]

Adding words that I tried to search for this by, for next time 🙂 : prefetch, prefetching, pre-fetching, eagerly fetch.