#31 Prefs Generator should add trailing slash to website URL

[Jayesh Sheth]

The prefs.php generator (setup/makeprefs.html) should
use JavaScript to check whether the URL of the website
specified as where Rilke CMS is installed ends with a
slash. If it does not, it should automatically add a slash.

This will prevent the occurence of a problem where
attached images do not show up because the value of
$titleurl in prefs.php does not have a trailing slash.