Bug: ReferenceError: assignment to undeclared variable child_node

[cbeer]

Description

When you try to nest feature page, there's a javascript error:

ReferenceError: assignment to undeclared variable child_node in updateWeightsAndRelationships

Steps to reproduce

Go to the feature page admin (e.g. http://127.0.0.1:3000/test-exhibit/feature). Make two feature pages. Try to nest one under the other. See the javascript error, and see that saving changes doesn't.

Expected behavior

It allows curators to nest pages.

[taylor-steve]

Might be worth checking if #3055 solves this problem, we need to move off of nestable anyway.