emacs: add notmuch-logo.svg and use it in emacs mua on graphic displays

emacs/notmuch-logo.svg is handcrafted scalable vector graphics version
of the notmuch logo.

Emacs on graphic displays render this image four times in size compared
to the emacs/notmuch-logo.png, and the image is much sharper.

The rendered image size, 100x100 pixels, fits reasonably well with
text shown in various font sizes.

Scaling the image -- if desired -- may come in the future. For now
this is improvement on how the logo looked in notmuch-hello window.