export ASSET_HOST to window.ENV so handlebars css loads

in order to serve the css for handlebars files
from a CDN this needs to be set.

Test Plan:

load a page that has handlebars css on it
see that it works

Change-Id: Ie9fec018e2d9fc95e1bb5c1a61b504797fb535a0
Reviewed-on: https://gerrit.instructure.com/60344
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Derek Hansen <dhansen@instructure.com>
Tested-by: Jenkins
Product-Review: Ryan Shaw <ryan@instructure.com>

app/controllers/application_controller.rb

@@ -109,6 +109,7 @@ def js_env(hash = {})
     unless @js_env
       editor_css = view_context.stylesheet_path(css_url_for('what_gets_loaded_inside_the_tinymce_editor'))
       @js_env = {
+        ASSET_HOST: asset_host,
         active_brand_config: active_brand_config.try(:md5),
         url_to_what_gets_loaded_inside_the_tinymce_editor_css: editor_css,
         current_user_id: @current_user.try(:id),