Add contract signer and contract signer role in the company definition.

Required SQL upgrade:

CREATE TABLE IF NOT EXISTS `contract_signer_role` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `role` varchar(32) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `role` (`role`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

SET character_set_client = utf8;
INSERT INTO contract_signer_role (role) VALUES
  ('Gérant'),
  ('Président')
  ;

ALTER TABLE webfinance_clients ADD COLUMN `contract_signer` varchar(100);
ALTER TABLE webfinance_clients ADD COLUMN `id_contract_signer_role` int(11);