cubicweb #1698261 ignore __message variable in GET or POST [open]

After using _cwmsgid instead of __message, http://www.cubicweb.org/ticket/1698245 the next step (when we are sure this doesn't break existing cubes) is to ignore the __message variable all together, and rely only on cwmsgid.

prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>