Use node packages to install Foundation

Since Foundation hasn't released a new gem for years, we haven't been
able to upgrade Foundation to its most recent version.

Thanks to this change, we'll be able to do so.

Note we're using motion-ui version 2.0.3 because version 2.0.5 (the
latest at the moment) requires Dart Sass.

app/assets/javascripts/application.js

@@ -51,7 +51,7 @@
 //= require jquery-ui/ui/widgets/autocomplete
 //= require jquery-ui/ui/widgets/sortable
 //= require jquery-fileupload/basic
-//= require foundation
+//= require foundation-sites
 //= require turbolinks
 //= require turbolinks_anchors
 //= require ckeditor/loader