Refactor containers to share more code (#61)

Specifically, make sure that the header, footer, and tabs are all
shared code so that they look the same and don't need to be
redefined as we add more tab pages.

src/course-header/Header.jsx

@@ -68,7 +68,13 @@ export default function Header({
 }
 
 Header.propTypes = {
-  courseOrg: PropTypes.string.isRequired,
-  courseNumber: PropTypes.string.isRequired,
-  courseTitle: PropTypes.string.isRequired,
+  courseOrg: PropTypes.string,
+  courseNumber: PropTypes.string,
+  courseTitle: PropTypes.string,
+};
+
+Header.defaultProps = {
+  courseOrg: null,
+  courseNumber: null,
+  courseTitle: null,
 };