emacs: tree: default face for matching/non-matching messages

This adds default faces for matching and non-matching messages. This
makes it easier for a user to do broad customization without having to
customize every field. It also fits more neatly with the next patch
which switches to using notmuch-tag-format-tags for tag formatting.

We set the field specific face customization to nil for all the fields
which use the message default face to make it clear to a user which
fields customizations are being used.