[request] Make sure set_message() actually displays its given message (closes #3003425) The default automatic form sets a standard submit message as session data. However in some cases, such as the cancellation of a form edition, another message is intended to be printed on-screen. This patch makes sure that set_message() really scrubs all previous messages (and ids) from the request object and from the session data.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)