Make an actual foreign key between docs versions and core versions

This was originally not done because we had a lot of old legacy data
that didn't have entries in both tables. This is no longer the case,
and the docloads script already enforced that it couldn't happen again.

Requires SQL:
ALTER TABLE core_version ADD CONSTRAINT version_unique_tree UNIQUE(tree);
ALTER TABLE docs ADD CONSTRAINT docs_version_fkey FOREIGN KEY (version) REFERENCES core_version(tree);