emacs: Adapt the logo background colour to that of the frame

The notmuch logo uses transparency. That can display poorly when
inserting the image into an emacs buffer (black logo on a black
background), so force the background colour of the image. We use a
face (`notmuch-hello-logo-background') to represent the colour so that
`defface' can be used to declare the different possible colours, which
depend on whether the frame has a light or dark background.