Move whatsapp_send to its own file

Move whatsapp_send to its own file

The send of whatsapp_send is now the function whatsappSend in
whatsapp_send.go, with its check of the arguments and its constants. It
owns no state: it gets a WhatsAppLink for the device, the client, the
connection, and the own IDs, the WhatsAppGroups for the name of a group
in the answer, and the state file for the send records. WhatsApp.Send
stays the entry point of the tool and gives the current link. The tests
of the tool move to whatsapp_send_test.go. The behavior does not change.

Issue: 6f69ca1