Add uniqueness constraint for meta_taggings

As suggested by redsummernight in the PR for AO3-2452 (in this comment), it would be good to add a unique index on (meta_tag_id, sub_tag_id) to the meta_taggings table. There’s already a constraint in ruby to try to prevent duplicate MetaTaggings, but it would be better to enforce that constraint in the database, as well.