Inject theming hooks into navigation bar

Allow themes to inherit from the default navigation bar and override
pieces of it, including the main logo, the links that display to the
right of the logo, and the links inside the dropdown menu (with the
exception of the `Log Out` link.

In addition, this adds an empty block at the very top so that themes
can place a branding bar at the top of the page. (Stanford identity
guidelines require this: see https://identity.stanford.edu.)

lms/templates/navigation.html

 ## mako
 <%namespace name='static' file='static_content.html'/>
-<%namespace file='main.html' import="login_query"/>
+<%namespace file='main.html' import="login_query, stanford_theme_enabled"/>
 <%!
 from django.core.urlresolvers import reverse
 
@@ -10,6 +10,9 @@ import branding
 from status.status import get_site_status_msg
 %>
 
+## Provide a hook for themes to inject branding on top.
+<%block name="navigation_top" />
+
 <%block cached="False">
 <%
 try:
@@ -38,9 +41,12 @@ site_status_msg = get_site_status_msg(course_id)
   <nav>
 
   <h1 class="logo">
-      <a href="${marketing_link('ROOT')}">
+    <a href="${marketing_link('ROOT')}">
+      <%block name="navigation_logo">
         <img src="${static.url(branding.get_logo_url(request.META.get('HTTP_HOST')))}" alt="edX home" />
-      </a></h1>
+      </%block>
+    </a>
+  </h1>
 
     % if course:
       <h2><span class="provider">${course.org}:</span> ${course.number} ${course.display_name_with_default}</h2>
@@ -49,9 +55,11 @@ site_status_msg = get_site_status_msg(course_id)
     % if user.is_authenticated():
 
     <ol class="left nav-global authenticated">
-      <li class="nav-global-01">
-      <a href="${marketing_link('COURSES')}">Find Courses</a>
-      </li>
+      <%block name="navigation_global_links_authenticated">
+        <li class="nav-global-01">
+          <a href="${marketing_link('COURSES')}">Find Courses</a>
+        </li>
+      </%block>
     </ol>
     <ol class="user">
       <li class="primary">
@@ -63,10 +71,9 @@ site_status_msg = get_site_status_msg(course_id)
       <li class="primary">
         <a href="#" class="dropdown">&#9662</a>
         <ul class="dropdown-menu">
-            <li>
-            <a href="${marketing_link('FAQ')}">
-            Help
-          </a></li>
+          <%block name="navigation_dropdown_menu_links" >
+            <li><a href="${marketing_link('FAQ')}">Help</a></li>
+          </%block>
           <li><a href="${reverse('logout')}">Log Out</a></li>
         </ul>
       </li>
@@ -74,17 +81,19 @@ site_status_msg = get_site_status_msg(course_id)
 
     % else:
     <ol class="left nav-global">
-      % if settings.MITX_FEATURES.get('ENABLE_MKTG_SITE'):
-      <li class="nav-global-01">
-        <a href="${marketing_link('HOW_IT_WORKS')}">How it Works</a>
-      </li>
-      <li class="nav-global-02">
-        <a href="${marketing_link('COURSES')}">Courses</a>
-      </li>
-      <li class="nav-global-03">
-        <a href="${marketing_link('SCHOOLS')}">Schools</a>
-      </li>
-      % endif
+      <%block name="navigation_global_links">
+        % if settings.MITX_FEATURES.get('ENABLE_MKTG_SITE'):
+          <li class="nav-global-01">
+            <a href="${marketing_link('HOW_IT_WORKS')}">How it Works</a>
+          </li>
+          <li class="nav-global-02">
+            <a href="${marketing_link('COURSES')}">Courses</a>
+          </li>
+          <li class="nav-global-03">
+            <a href="${marketing_link('SCHOOLS')}">Schools</a>
+          </li>
+        % endif
+      </%block>
       % if not settings.MITX_FEATURES['DISABLE_LOGIN_BUTTON']:
       <li class="nav-global-04">
         <a class="cta cta-register" href="/register">Register Now</a>