Properly unlink all old avatars when deleting/uploading a new

We're also now using $config['image']['jpegquality'] to determine the
quality setting for resized images.

To set Avatar max size, adjust $config['avatar']['maxsize']

The getAvatar call now throws exceptions too. Related changes applied.
Now let's move Profile->avatarUrl to the Avatar class!

classes/Memcached_DataObject.php

@@ -276,7 +276,6 @@ class Memcached_DataObject extends Safe_DataObject
             throw new NoResultException($i);
         }
 
-        sprintf(__CLASS__ . "() got {$i->N} results for class $cls key $keyCol");
         return $i;
     }