emacs: add notmuch-show-refresh-view function

emacs: add notmuch-show-refresh-view function

This function, like the equivalent for notmuch-search, just refreshes
the current show view.  Like in notmuch-search, this new function is
bound to "=".  If a prefix is given then the redisplay happens with the
crypto-switch set, which displays the thread with the opposite logic
of whatever is set in the notmuch-crypto-process-mime customization
variable.