Make edX logo in edx.org footer more accessible

The logo in the edx.org footer now links to the edX home page, consistent with the logo in the header. ECOM-4117.

themes/edx.org/lms/templates/footer.html

@@ -18,7 +18,9 @@
     <h2 class="sr footer-about-title">${_("About edX")}</h2>
     <div class="footer-content-wrapper">
       <div class="footer-logo">
-          <img alt="edX logo" src="${footer['logo_image']}">
+          <a href="${marketing_link('ROOT')}">
+            <img alt="edX Home Page" src="${footer['logo_image']}">
+          </a>
       </div>
 
       <div class="site-details">