Enforce use of bigserial for primary keys with rubocop

We want to avoid running into integer overflows with sequences. That is, we want to be using bigserial by default when creating new tables. Let's add a rubocop check to enforce this.

In cases where we're sure we don't need a bigint, we can simply ignore the rubocop rule. By default, the rubocop check will serve well to trigger a discussion about it though.