Add support for video download link

Add a download link to the video player. The link is defined in the
content xml using the following structure:

<video youtube="youtubestring">
  <source src="video_link"/>
</video>

common/lib/xmodule/xmodule/video_module.py

@@ -31,12 +31,23 @@ class VideoModule(XModule):
         self.youtube = xmltree.get('youtube')
         self.position = 0
         self.show_captions = xmltree.get('show_captions', 'true')
+        self.source = self._get_source(xmltree)
 
         if instance_state is not None:
             state = json.loads(instance_state)
             if 'position' in state:
                 self.position = int(float(state['position']))
 
+    def _get_source(self, xmltree):
+        # find the first valid source
+        source = None
+        for element in xmltree.findall('source'):
+            src = element.get('src')
+            if src:
+                source = src
+                break
+        return source
+
     def handle_ajax(self, dispatch, get):
         '''
         Handle ajax calls to this video.
@@ -73,6 +84,7 @@ class VideoModule(XModule):
             'streams': self.video_list(),
             'id': self.location.html_id(),
             'position': self.position,
+            'source': self.source,
             'display_name': self.display_name,
             # TODO (cpennington): This won't work when we move to data that isn't on the filesystem
             'data_dir': self.metadata['data_dir'],
@@ -82,6 +94,5 @@ class VideoModule(XModule):
 
 class VideoDescriptor(RawDescriptor):
     module_class = VideoModule
-    
     stores_state = True
     template_dir_name = "video"

lms/templates/video.html

@@ -13,3 +13,8 @@
     </article>
   </div>
 </div>
+% if source:
+<div class="video-sources">
+  <p>Download video <a href="${source}">here</a>.</p>
+</div>
+% endif