feat: handle courseware paths more liberally (#395)

Valid courseware URLs currently include:
* /course/:courseId
* /course/:courseId/:sequenceId
* /course/:courseId/:sequenceId/:unitId

In this commit we add support for:
* /course/:courseId/:sectionId
* /course/:courseId/:sectionId/:unitId
* /course/:courseId/:unitId

All URL forms still redirect to:
  /course/:courseId/:sequenceId/:unitId

See ADR #8 for more context.

All changes:
* refactor: allow courseBlocks factory to build multiple sections
* refactor: make CoursewareContainer tests less brittle & stateful
* feat: handle courseware paths more liberally
* refactor: reorder, rename, & comment redirection functions

TNL-7796

src/courseware/data/thunks.js

@@ -93,15 +93,26 @@ export function fetchSequence(sequenceId) {
     dispatch(fetchSequenceRequest({ sequenceId }));
     try {
       const { sequence, units } = await getSequenceMetadata(sequenceId);
-      dispatch(updateModel({
-        modelType: 'sequences',
-        model: sequence,
-      }));
-      dispatch(updateModels({
-        modelType: 'units',
-        models: units,
-      }));
-      dispatch(fetchSequenceSuccess({ sequenceId }));
+      if (sequence.blockType !== 'sequential') {
+        // Some other block types (particularly 'chapter') can be returned
+        // by this API. We want to error in that case, since downstream
+        // courseware code is written to render Sequences of Units.
+        logError(
+          `Requested sequence '${sequenceId}' `
+          + `has block type '${sequence.blockType}'; expected block type 'sequential'.`,
+        );
+        dispatch(fetchSequenceFailure({ sequenceId }));
+      } else {
+        dispatch(updateModel({
+          modelType: 'sequences',
+          model: sequence,
+        }));
+        dispatch(updateModels({
+          modelType: 'units',
+          models: units,
+        }));
+        dispatch(fetchSequenceSuccess({ sequenceId }));
+      }
     } catch (error) {
       logError(error);
       dispatch(fetchSequenceFailure({ sequenceId }));