move old messages out of memory

this thing will just keep sucking up messages and eating up memory as long as you leave it open. it also gets increasingly stressful for react to render larger conversations. (improving `MessageList` rendering significantly improved react performance, but switching to a long conversation still hangs slightly)

in the short term, it'd be easy to cap messages per convo and just forget about old messages.

eventually, it'd be nice if this cached old messages to disk. we'd have to coordinate reading them in as you scrolled and such, which would take some work. rejoining a channel and seeing messages from when you were last online would be cool tho

Start by tracing MessageList and the conversation message state to understand where messages accumulate and are rendered. Define the short-term cap for stored messages and verify that long conversations no longer retain or render an unbounded history; disk caching and loading while scrolling are described as future work.

Written by the indexing model from the issue text.