"Select parents" should use find selector to locate children checkboxes

When checking a child, the parents are automatically selected. When this happens, the code looks from the parent and checks all the boxes down to the child. To do this, is uses this code:

checkbox.parents('ul.term-reference-tree-level li').children('div.form-item').children('input[type=checkbox]').each(function() {

However, this is too brittle. For instance, bootstrap puts checkboxes within label elements. So, we should use find for the inputs, not children. e.g.

checkbox.parents('ul.term-reference-tree-level li').children('div.form-item').find('input[type=checkbox]').each(function() {

Patch attached.