Make viewport a module that caches height and width values

The .height() and .width() functions are actually pretty expensive for
the number of times we call them.  The viewport height and width
don't change often, though, so we can just cache them and recalculate
them on window resize.

(imported from commit 129fb8c058144125e2974f6b7967cd9f1a5c9ead)

zephyr/static/js/zephyr.js

@@ -9,8 +9,6 @@ var recent_subjects = {};
 var queued_mark_as_read = [];
 var queued_flag_timer;
 
-var viewport = $(window);
-
 var get_updates_params = {
     pointer: -1
 };