Issues and projects gained a `_links` field containing URLs of related
APIs (see gitlab-ce MR 10491, commit v9.5.0-rc1~301^2).  We model it
but do not expose it publicly since our clients won't need it.

Notes gained a `noteable_iid` when attached to issues and merge
requests (see gitlab-ce MR 13265, commit v9.5.0-rc1~14^2).