emacs: sanitize function that displays version

emacs: sanitize function that displays version

Previously it was defined in "notmuch-hello.el" and its name contained
"hello" solely because it replaced an anonymous function that was
mistakenly only bound in `notmuch-hello-mode-map'.  But it makes more
sense to bind it in all notmuch modes and even if we did not change
that aspect it still would make no sense to have "hello" in its name.