dev/core#2271 - APIv4 - Smarter check for adding default ON clause to explicit joins

dev/core#2271 - APIv4 - Smarter check for adding default ON clause to explicit joins

When adding explicit joins in APIv4, the API will automacially add an ON clause linking it
to the base entity unless it detects some other link already present in the ON clause supplied.
Previously it would determine this by checking for the join entity ID;
now it looks for the join entity ID OR any other field in the join entity with an FK.
This gives more leeway for people to add creative joins without the defaults getting in the way.