Permit to skip message escaping in flash messages

[amol-]

From mailing list it has been reported the need to place links into the flash messages, while this is not always desiderable it might make sense in some cases.

Suggested implementation on ML was:

def _render_static_version(self, container_id):
        payload = self.pop_payload()
        if not payload:
            return ''

        # HACK - THIS IS A PATCH
        if payload.get('no_escape'):
            payload['message'] = payload.get('message','')
        else:
            payload['message'] = escape(payload.get('message',''))

        payload['container_id'] = container_id
        return self.static_template.substitute(payload)

[amol-]

This should be covered by 4e4bdea