Add more modern customization options, fix the subscription link/button

This adds a couple of new features that have been available since Angua:

* The use of ``tpl_favicon`` makes it possible to easily replace the favicon by uploading a media file
* The ``tpl_includeFile``-function allows placing the included files in the conf directory which makes template customization easier for farm setups and allows to replace ``footer.html`` without modifying the template

Furthermore the "subscription" action has been renamed to "subscribe" even earlier and there were problems with the check if subscriptions are enabled with the old action name (the link was always displayed). I've also removed a superfluous ``?>`` at the end of tpl_functions.php.