Fix images paths in articles template

Fix paths suing site url to compute paths for interface images -
hardcoded paths fails on some servers probably due to different
.htaccess configuration details:
Two of three interface images have used: 
````
../../assets/images/icons/interface/back.png
````
changed to path build that way:
````
<?php echo html($site->url()) ?>/assets/images/icons/interface/back.png
````