plugin tickets: handle "update" flag + various improvments: - ticketsSet renamed to ticketSet as only one ticket is set at a time - dateFilter is not crashing anymore if value can't be parsed: the value is then return unmodified - form_ns can be specified in get, so the method can be used by other plugins - new "update" flag: if set in extra, an previous ticket of the same id will be retrieved, and used as a basis for an update