fix: use new xblock handler URLs for sequence blocks (#423)

They've changed to proper new-style handlers, so the URL also
changed. This will let us get the fix for green checkmarks showing
up as audit users pass FBE units, even though those units aren't
actually complete.

AA-409

src/courseware/CoursewareContainer.test.jsx

@@ -374,7 +374,7 @@ describe('CoursewareContainer', () => {
       });
 
       it('should navigate between units and check block completion', async () => {
-        axiosMock.onPost(`${courseId}/xblock/${sequenceBlock.id}/handler/xmodule_handler/get_completion`).reply(200, {
+        axiosMock.onPost(`${courseId}/xblock/${sequenceBlock.id}/handler/get_completion`).reply(200, {
           complete: true,
         });