Add hooks to override default edX header

Adds two hooks:

1. Allows Stanford brand bar to go on top of the header
2. Allows Stanford to replace main site logo with its own

lms/templates/navigation.html

@@ -10,6 +10,8 @@ import branding
 from status.status import get_site_status_msg
 %>
 
+<%block name="navigation_top"/>
+
 <%block cached="False">
 <%
 try:
@@ -37,10 +39,13 @@ site_status_msg = get_site_status_msg(course_id)
 % endif
   <nav>
 
-  <h1 class="logo">
+    <h1 class="logo">
       <a href="${marketing_link('ROOT')}">
-        <img src="${static.url(branding.get_logo_url(request.META.get('HTTP_HOST')))}"/>
-      </a></h1>
+        <%block name="navigation_logo">
+          <img src="${static.url(branding.get_logo_url(request.META.get('HTTP_HOST')))}"/>
+        </%block>
+      </a>
+    </h1>
 
     % if course:
       <h2><span class="provider">${course.org}:</span> ${course.number} ${course.display_name_with_default}</h2>