pointer: Merge pointer.js into message_scroll.js.

Ths remaining code here is naturally message scrolling logic, and fits
well into the existing file for handling that.

static/js/ui_init.js

@@ -471,7 +471,6 @@ exports.initialize_everything = function () {
     settings_sections.initialize();
     settings_toggle.initialize();
     hashchange.initialize();
-    pointer.initialize();
     unread_ui.initialize();
     presence.initialize(presence_params);
     activity.initialize();