wl-draft-mode: Use rebind to map wl-draft-highlight-and-recenter

If using C-l as a prefix key Wanderlust's draft mode won't work as it is
trying to bind C-l to wl-draft-highlight-and-recenter.  By rebinding the
default function associated with C-l, recenter-top-bottom, the key can
be used as a prefix and still work with Wanderlust.