use :cdn_host from env (fixes philomena-dev/philomena#54)

lib/philomena_web/templates/layout/_flash_warnings.html.slime

@@ -10,7 +10,7 @@
 noscript.flash.flash--warning
   strong You don't appear to have Javascript enabled
   ' If you're using an add-on like NoScript, please allow
-  ' :cdn_host
+  => cdn_host()
   ' for the site to work properly.
 = if get_flash(@conn, :info) do
   .flash.flash--success = get_flash(@conn, :info)

lib/philomena_web/views/layout_view.ex

@@ -18,6 +18,10 @@ defmodule PhilomenaWeb.LayoutView do
     |> Float.to_string()
   end
 
+  def cdn_host do
+    Application.get_env(:philomena, :cdn_host)
+  end
+
   defp ignored_tag_list(nil), do: []
   defp ignored_tag_list([]), do: []
   defp ignored_tag_list([{tag, _body, _dnp_entries}]), do: [tag.id]