notmuch.el: Make 'x' and 'X' in show-mode archive the current thread.

This makes these keys different than 'q' in this mode, (where 'x'
and 'q' are identical in all of the other modes currently).

The idea here is to make it easier to do non-linear reading of messages,
(such as when poking in to read just one or two threads from a search
result that returned many threads).