emacs: `notmuch' should display the `notmuch-hello' interface

Reviewed-by: Carl Worth <cworth@cworth.org>
The notmuch-hello functionality is now sufficiently useful that we
want to make it the default view of notmuch for new users. This also
effectively hides the "hello" name from the user, so we'll be free to
change that in the implementation if necessary.

This change also shuffles the requires between notmuch.el and
notmuch-hello.el. This fixes things so that our documented (require
'notmuch) is sufficient for getting the notmuch-hello functionality.

Finally, the shuffling caused the notmuch-search-oldest-first variable
from one file to the other. While doing that, give this variable the
defcustom treatment for easier customization.