emacs: replace use of notmuch-address-message-insinuate

emacs: replace use of notmuch-address-message-insinuate

This allows e.g. Gnus users to load this file without changing
message-mode behaviour.

This will disable completion for those that did not customize the
variable but relied on the existence of a file named "notmuch-addresses"
in their path. In the next commit the default behaviour will change to
use a "workalike" internal completion mechanism.