fix: show correct icons in the sidebar for units with custom XBlocks

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.

lms/djangoapps/course_api/blocks/serializers.py

@@ -59,6 +59,7 @@ SUPPORTED_FIELDS = [
     SupportedFieldType('weight'),
     SupportedFieldType('show_correctness'),
     SupportedFieldType('hide_from_toc'),
+    SupportedFieldType('icon_class'),
     # 'student_view_data'
     SupportedFieldType(StudentViewTransformer.STUDENT_VIEW_DATA, StudentViewTransformer),
     # 'student_view_multi_device'

lms/djangoapps/course_api/blocks/transformers/blocks_api.py

@@ -54,7 +54,15 @@ class BlocksAPITransformer(BlockStructureTransformer):
         transform method.
         """
         # collect basic xblock fields
-        block_structure.request_xblock_fields('graded', 'format', 'display_name', 'category', 'due', 'show_correctness')
+        block_structure.request_xblock_fields(
+            'graded',
+            'format',
+            'display_name',
+            'category',
+            'due',
+            'show_correctness',
+            'icon_class',
+        )
 
         # collect data from containing transformers
         StudentViewTransformer.collect(block_structure)