Add a Character.other_characters property.
This property allows other involved Characters to be attached to an
Influence.
SQL: CREATE TABLE `influences_influence_other_characters` (
SQL: `id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
SQL: `influence_id` integer NOT NULL,
SQL: `character_id` integer NOT NULL,
SQL: UNIQUE (`influence_id`, `character_id`)
SQL: );
SQL: ALTER TABLE `influences_influence_other_characters` ADD
SQL: CONSTRAINT influence_id_refs_id_f981877b32ddc18
SQL: FOREIGN KEY (`influence_id`)
SQL: REFERENCES `influences_influence` (`id`);
SQL: ALTER TABLE `influences_influence_other_characters` ADD
SQL: CONSTRAINT character_id_refs_id_56216395feac9718
SQL: FOREIGN KEY (`character_id`)
SQL: REFERENCES `influences_character` (`id`);