Added sendDraft and refactored to use it instead of send.

All interactions with pond.Message from cli.go and gui.go now take place in network.go.  The sendDraft func isolates updating the ratchet.

I believe this should simplify future code to send messages to multiple recipients, and reduce the chances of making mistakes when writing future UIs.

This should also fix a minor hickup in the GUI where, if the user begins a reply, but deletes the inbox message before replying, then the reply loses its status as a reply.