Cosmetic improvements to course materials page

* All child items indent to the same (2em) level regardless of type
  (fixes #1799)
* file_list_item.html and additional_page_list_item.html get their icons
  toggled by the is_child variable.
* file_list_item.html and additional_page_list_item.html used for
  rendering child files and pages in ready-mode course materials view,
  so children render (almost) just like their parents.
  (fixes #1808)
Merge pull request #1814 from Stanford-Online/jrbl-1808-child-items-like-parents

Cosmetic improvements to course materials page