Automatic handling of AuthRemoteTooLate

[multimeric]

FileSender has a mechanism whereby any request that is received more than 15 seconds after the timestamp header on that same request is considered "too late". This generally happens with a slow connection. It should be possible to catch these errors and adjust the delay parameter automatically to avoid these errors without needing user intervention.

Relevant source code:
https://github.com/filesender/filesender/blob/4e618aea586b5dfad647945f50ddbdaf3d4c4a98/classes/auth/AuthRemote.class.php#L105-L109