emacs: remap send-message and send-message-and-exit

emacs: remap send-message and send-message-and-exit

All three of C-c C-c, <menu-bar> <Message> <Send Message>,
and <tool-bar> <Send Message> are bound to message-send-and-exit by
message.el, but notmuch-mua.el only had an explicit override for the
keyboard binding. This mostly manifests as confusing Fcc behaviour for
GUI users.

Patching the bindings for specific keys is rather brittle, since it has
to be aware of every relevant binding. This change switches to instead
using a remap binding, which turns any binding for message-send or
message-send-and-exit into a binding for the corresponding notmuch-mua
command.