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`);