Set and use ETag headers

Also set no-cache exactly as github recommends. The removed @no_cache
decorater used "Cache-Control:no-cache, no-store, must-revalidate", but
just no-cache should be sufficient, and should certainly work correctly
with github.

See: https://github.com/github/markup/issues/224#issuecomment-48532178

fixes #712

endpoints/web.py

@@ -399,7 +399,6 @@ def confirm_recovery():
 
 @web.route('/repository/<path:repository>/status', methods=['GET'])
 @parse_repository_name
-@no_cache
 @anon_protect
 def build_status_badge(namespace, repository):
   token = request.args.get('token', None)
@@ -423,8 +422,13 @@ def build_status_badge(namespace, repository):
   else:
     status_name = 'none'
 
+  if request.headers.get('If-None-Match') == status_name:
+    return Response(status=304)
+
   response = make_response(STATUS_TAGS[status_name])
   response.content_type = 'image/svg+xml'
+  response.headers['Cache-Control'] = 'no-cache'
+  response.headers['ETag'] = status_name
   return response