Harmonize, clarify, categorize URL schemes

Regular expression + avoid-redirection list now match each other.

classes/File_redirection.php

@@ -312,15 +312,20 @@ class File_redirection extends Managed_DataObject
                 $out_url = str_replace('.-()', '', $out_url);
                 break;
 
+            // non-HTTP schemes, so no redirects
             case 'bitcoin':
             case 'mailto':
-            case 'magnet':
             case 'aim':
             case 'jabber':
             case 'xmpp':
                 // don't touch anything
                 break;
 
+            // URLs without domain name, so no redirects
+            case 'magnet':
+                // don't touch anything
+                break;
+
             default:
                 $out_url = $default_scheme . ltrim($out_url, '/');
                 $p = parse_url($out_url);