frontend: Implement list_render class.

This implements a list_render closure class that allows for
progressive, responsive rendering of long, scrollable lists, with
filtering support.

It isn't used, at present.

zproject/settings.py

@@ -862,6 +862,7 @@ JS_SPECS = {
             'js/message_edit.js',
             'js/condense.js',
             'js/resize.js',
+            'js/list_rendering.js',
             'js/floating_recipient_bar.js',
             'js/lightbox.js',
             'js/ui_state.js',