Initial load via burst of socket messages, deprecate /get/chats

Upon loading Meatspace, perform the initial sync of recent chat messages via the existing web socket message emission rather than via a large GET. Messages stream in one by one through the socket instead of a large ~3.5MB payload. Note, this deprecates the `HTTP GET /get/chats` endpoint, so native clients will have to be updated.

The advantages of this are:

  * Messages are streamed out one by one and start arriving immediately, providing a more snappy initial load instead of a huge loading delay and blank screen.
  * Flatten both ways of receiving chat messages into one method, yielding less code.
  * Finer-grained arrival of the initial messages means the CPU can yield to the UI during the initial transfer, helping weaker devices survive the first paint.