Make wl-highlight-message use the citation depth for choosing the face.

Rather than using a stateful algorithm for choosing arbitrary but
distinct faces for the various citation prefixes in the message,
this uses the citation depth for plain citations and an arbitrary
hash of the attribution name for SC-style attributions.
Implement draft highlighting using jit-lock-mode.

This uses Emacs' 21 native highlighting mechanism, and is enabled
by default if available.