[5.0] Eloquent foreign key getter

[tedslittlerobot]

As described in #7162.

In my experience, whenever i've overridden the $table property, it has always made more sense for foreign keys pointing to that model to reflect the specified table name, rather than the class name.

(i've put this as 5.0, because although it could be implemented in 4.*, it would break b/c)

[taylorotwell]

Don't want to break BC here.