Add a Characters.contacts propety.

This property lists all the other characters that are contacts of a
Character. The property is symmetrical, so if A is a contact of B, the
reverse is also automatically the case.

SQL: CREATE TABLE `influences_character_contacts` (
SQL:    `id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
SQL:    `from_character_id` integer NOT NULL,
SQL:    `to_character_id` integer NOT NULL,
SQL:    UNIQUE (`from_character_id`, `to_character_id`)
SQL: );
SQL: ALTER TABLE `influences_character_contacts` ADD
SQL:   CONSTRAINT from_character_id_refs_id_653bf18d29c7a4fa
SQL:   FOREIGN KEY (`from_character_id`)
SQL:   REFERENCES `influences_character` (`id`);
SQL: ALTER TABLE `influences_character_contacts` ADD
SQL:   CONSTRAINT to_character_id_refs_id_653bf18d29c7a4fa
SQL:   FOREIGN KEY (`to_character_id`)
SQL:   REFERENCES `influences_character` (`id`);