Make VersionClass.versions a plain list#332

This (VersionClass.versions) uses a `lazy='dynamic'` relationship, which is on its way out: ``` first_version = article.versions[0] ``` This also supports negative indexes, which a lot of the tests use. They were never performant and [are removed in SQLAlchemy 2.0](https://github.com/sqlalchemy/sqlalchemy/issues/5606). While we could create an accessor that uses `order_by` to do the right thing, it's probably less surprising to just make `.versions` return a list instead of using `lazy='dynamic'`. We could expose a separate helper to get particular versions by index or version count. Do people think this is useful?