Note from tabbott: This is a somewhat surprising feature to be adding
this late in Zulip's development, but the model we've had of what
narrows are easy to access via clicking around has meant that it was
fairly difficult to get into a narrow that didn't include the very
latest messages in that narrow.
Fixes#3465.
We are going to remove message_fetch.reset_for_narrow() soon,
but its callers probably still want to hide any scrolling
indicators, and we just let them do that directly.
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