Some small url fixes

This pull fixes:

* The default frontend theme (when chosing no theme in backend) could not load the Head.tpl and Footer.tpl.
* Fix: favicon.ico cannot be loaded.  When going to this page: /src/Api/V1/docs/index.htm you will find that the favicon.ico cannot be loaded.
* Fix: ds.png image cannot be loaded.  When going to this backend page: /private/nl/form_builder/edit?token=xxxxxxxxx&id=1 you will find that the ds.png image cannot be loaded.
* Fix: spinner.gif image cannot be loaded.  When going to this backend page: /private/nl/settings/email you will find that the spinner.gif image cannot be loaded.