Currently, the sidebar relies only on the XBlock's `category` class attribute
(called `type` in the transformers). This behavior is inconsistent with the
legacy subsection navigation, which relies on the `XModuleMixin.get_icon_class`
method. This commit adds the `icon_class` to the fields collected by the
transformers and uses it to determine whether the "problem" or "video" icon
should be displayed for a unit in the sidebar.
bbc0cc2baa