TNL-7299 - Hide sequences that are time limited (special exams) (#241)

If a sequence has its isTimeLimited flag set, then show a spinner instead of the sequence content.  The CoursewareContainer, meanwhile, will be attempting to redirect to the legacy experience.

This prevents a situation where we temporarily show proctored/special exam content to users before their exam starts.

src/courseware/course/sequence/Sequence.jsx

@@ -113,6 +113,20 @@ function Sequence({
     );
   }
 
+  /*
+  TODO: When the micro-frontend supports viewing special exams without redirecting to the legacy
+  experience, we can remove this whole conditional. For now, though, we show the spinner here
+  because we expect CoursewareContainer to be performing a redirect to the legacy experience while
+  we're waiting. That redirect may take a few seconds, so we show the spinner in the meantime.
+  */
+  if (sequenceStatus === 'loaded' && sequence.isTimeLimited) {
+    return (
+      <PageLoading
+        srMessage={intl.formatMessage(messages['learn.loading.learning.sequence'])}
+      />
+    );
+  }
+
   const gated = sequence && sequence.gatedContent !== undefined && sequence.gatedContent.gated;
   const goToCourseExitPage = () => {
     history.push(`/course/${courseId}/course-exit`);