Configurable icons for external sites app

This extends the external app, to choose a icon for each external URL.
The icon must be added to the external/img folder manually.

There is always an empty form displayed in the admin area now. This saves the admin one click and is necessary to copy an existing li element (containing the icon selection list) when Add is clicked. It also keeps the HTML code in the template only.
The only problem with that: If one removes the last form, he can not add new ones until site reload.

Please let us discuss.

P.S.: And I added a small bug-fix to allow URLs beginning with / again, as this is a valid scenario with Apache mod_proxy.