Extract message_scroll.js.

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.

static/js/message_scroll.js

@@ -0,0 +1,74 @@
+var message_scroll = (function () {
+
+var exports = {};
+
+var actively_scrolling = false;
+
+var loading_more_messages_indicator_showing = false;
+exports.show_loading_more_messages_indicator = function () {
+    if (! loading_more_messages_indicator_showing) {
+        loading.make_indicator($('#loading_more_messages_indicator'),
+                                    {abs_positioned: true});
+        loading_more_messages_indicator_showing = true;
+        floating_recipient_bar.hide();
+    }
+};
+
+exports.hide_loading_more_messages_indicator = function () {
+    if (loading_more_messages_indicator_showing) {
+        loading.destroy_indicator($("#loading_more_messages_indicator"));
+        loading_more_messages_indicator_showing = false;
+    }
+};
+
+exports.actively_scrolling = function () {
+    return actively_scrolling;
+};
+
+exports.scroll_finished = function () {
+    actively_scrolling = false;
+
+    if ($('#home').hasClass('active')) {
+        if (!pointer.suppress_scroll_pointer_update) {
+            message_viewport.keep_pointer_in_view();
+        } else {
+            pointer.suppress_scroll_pointer_update = false;
+        }
+        floating_recipient_bar.update();
+        if (message_viewport.scrollTop() === 0) {
+            message_fetch.load_more_messages({
+                msg_list: current_msg_list,
+                show_loading: exports.show_loading_more_messages_indicator,
+                hide_loading: exports.hide_loading_more_messages_indicator,
+            });
+        }
+
+        // When the window scrolls, it may cause some messages to
+        // enter the screen and become read.  Calling
+        // unread_ops.process_visible will update necessary
+        // data structures and DOM elements.
+        setTimeout(unread_ops.process_visible, 0);
+    }
+};
+
+var scroll_timer;
+function scroll_finish() {
+    actively_scrolling = true;
+    clearTimeout(scroll_timer);
+    scroll_timer = setTimeout(exports.scroll_finished, 100);
+}
+
+exports.initialize = function () {
+    message_viewport.message_pane.scroll($.throttle(50, function () {
+        unread_ops.process_visible();
+        scroll_finish();
+    }));
+};
+
+
+return exports;
+
+}());
+if (typeof module !== 'undefined') {
+    module.exports = message_scroll;
+}