getNestedConfig() called on the parent but should be called on the child

[GuySartorelli]

The current state means that a parent is responsible for its children's nested config. This isn't intuitive. Each class should be responsible for its own nested config (i.e. responsible for determining how to show its own children).

PRs

• FIX Make child responsible for its own nested gridfield #393

[sabina-talipova]

Issue was solved. Task is completed.

[GuySartorelli]

FYI @forsdahl I didn't pick up on this during the initial PR, but when looking at the problems Sabina was hitting getting this set up for behat tests I realised this would be a lot more intuitive.