emacs: Rework crypto switch toggle.

Re-work the existing crypto switch toggle to be based on a persistant
buffer-local variable.

To allow this, modify `notmuch-show-refresh-view' to erase and re-draw
in the current buffer rather than killing the current buffer and
creating a new one. (This will also allow more per-buffer behaviour in
future patches.)

Add a binding ('$') to toggle crypto processing of the current buffer
and remove the prefix argument approach that achieves a similar
result.

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