allow forward slashes in the bcrypt salt

The forward slash is a valid character in the encoding of the bcrypt salt.
As a consequence on some unit (the salt is derived from the unique id), the
salt was regarded as invalid and hence it was impossible to login.

index.php

@@ -13,7 +13,7 @@ function getSalt() {
 
 	// NOTE: be strict about the format, to prevent accidentally leaking secrets if
 	// a different format is used e.g.
-	if (!preg_match('/^\$2a\$08\$[A-Za-z0-9+\\.]{22}$/', $salt))
+	if (!preg_match('/^\$2a\$08\$[A-Za-z0-9+\\.\/]{22}$/', $salt))
 		return "";
 
 	return $salt;