Re-filter children from course materials

* Fixes bug that recent course_materials refactoring introduced, showing
  child items as both children and as top-level items. This introduces
  explicit filtering of children in the template to render
  course_materials, with explanatory comments.
  (fixes #2278)
Merge pull request #2288 from Stanford-Online/jrbl-2278-course_materials_refactoring_bugfix

Re-filter children from course materials