This effectively changes a condition from `=== 0` to `<= 0`,
but that probably doesn't matter (and would only help). More
importantly, it just makes the code more consistent.
This mostly moves code from ui.js.
We change the arguments to `message_fetch.load_more_messages()`
to be `opts` with callbacks for `show_loading` and `hide_loading`.
We also defer starting the scroll handler until `message_fetch.js`
has been initialized.
Steve Howell