notmuch.el: Allow for scrolling backwards through thread with DEL

notmuch.el: Allow for scrolling backwards through thread with DEL

Otherwise known as "Backspace" on keyboards in the real, (rather than
emacs), world. This will go by screenfuls for long messages, and
message by message for short messages. So it does the reverse of the
magic space bar, (but without reversing any tag-changing magic that
the magic space bar might have done).