emacs: Use the new JSON reply format and message-cite-original

emacs: Use the new JSON reply format and message-cite-original

Use the new JSON reply format to create replies in emacs. Quote HTML
parts nicely by using mm-display-part to turn them into displayable
text, then quoting them with message-cite-original. This is very
useful for users who regularly receive HTML-only email.

Use message-mode's message-cite-original function to create the
quoted body for reply messages. In order to make this act like the
existing notmuch defaults, you will need to set the following in
your emacs configuration:

message-citation-line-format "On %a, %d %b %Y, %f wrote:"
message-citation-line-function 'message-insert-formatted-citation-line

The tests have been updated to reflect the (ugly) emacs default.