DeadDrop

currently the attachment size is limited by the Web Server Max post body size. This shall be handled more elegant:
1. The server shall set the max upload size as a parameter value and send it to the client at login time.
2. The server shall be able to receive Message attachments in multiple (chunked) requests
2. The client shall chunk larger attachments and send multiple requests
3. larger attachments shall not be downloaded by the client by default --> the standard message request contains a link only
4- The server shall provide a function to download an attachment of a message
5. The client must indicate if there is a large attachment and provide a function to download it.