Set window.CKEDITOR_BASEPATH in text_plugin_change_form.html to fix #124 properly

The [previous pull request](https://github.com/divio/djangocms-text-ckeditor/pull/125) did not work if settings.STATIC_URL was a protocol-independent URL, e.g. //static.example.com/files.

This fixes 124 properly by adding the protocol to these URLs.