[manual] Fetch Handlebars templates using Ajax

...rather than embedding them into index.html.

This is only acceptable for dev, but the next commit adds an alternative
mechanism for prod.

There isn't actually a manual deployment step here.  However, this commit won't
work on staging / prod without the next one (since we don't serve
zephyr/static/templates in prod).

(imported from commit dce7ddfe89e07afc3a96699bb972fd124335aa05)

zephyr/static/js/message_list.js

@@ -307,7 +307,7 @@ MessageList.prototype = {
             this._message_groups = this._message_groups.concat(new_message_groups);
         }
 
-        var rendered_elems = $(templates.message({
+        var rendered_elems = $(templates.render('message', {
             messages: messages_to_render,
             include_layout_row: (table.find('tr:first').length === 0)
         }));