emacs: use new face for notmuch-jump and related

The minibuffer-prompt face that was used before made it impossible to
differentiate between two distinct UI elements: (i) the prompt's text
which itself cannot be acted upon, (ii) the actionable keys used to
jump to searches/tags.

The use of a named face, notmuch-jump-key, makes it possible for users
or theme developers to apply properties that are specific to each of
those two cases.

In the interest of backward compatibility, the new face inherits from
minibuffer-prompt.