Configuration of mail.

If a column that takes mail template id is 0, that will not be mailed.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
signature	TEXT	NOT NULL	Signature of mail. It would be written down on the end of each mail.
ordered	INTEGER	NOT NULL	Mail template ID for ordered information mail. It is mailed when a customer made order. (mail_template.id)
delivered	INTEGER	NOT NULL	Mail template ID for shipping information mail. It is mailed when a shopkeeper changed order.state to "delivered." (mail_template.id)
transfer_info	INTEGER	NOT NULL	Mail template ID for transfer information mail. It is mailed when a shopkeeper changed order.state to "wayting for a transfer." (mail_template.id)
canceling	INTEGER	NOT NULL	Mail template ID for cancel request information mail. It is mailed when a customer requested cancellation. (mail_template.id)
canceled	INTEGER	NOT NULL	Mail template ID for canceled information mail. It is mailed when a shopkeeper changed order.state to "canceled." (mail_template.id)

CREATE TABLE mail_setting (
shop          TEXT     PRIMARY KEY,
signature     TEXT     NOT NULL,
ordered       INTEGER  NOT NULL,
delivered     INTEGER  NOT NULL,
transfer_info INTEGER  NOT NULL,
canceling     INTEGER  NOT NULL,
canceled      INTEGER  NOT NULL
);