Fix static URL translation for templates rendered on the client side.

When rendering a fragment on the server side, there is magic in edX to translate
/static/... URLs to the actual file locations.  That magic does not apply for
URLs transmitted in AJAX responses and used in templates on the client side.
This fix translates the URLs in the JSON response before sending it back to the
client.

poll/poll.py

@@ -22,11 +22,13 @@
 #
 
 from collections import OrderedDict
+import functools
+import json
 
 from django.template import Template, Context
 from markdown import markdown
-
 import pkg_resources
+from webob import Response
 
 from xblock.core import XBlock
 from xblock.fields import Scope, String, Dict, List, Boolean, Integer
@@ -43,6 +45,12 @@ try:
 except ImportError:
     HAS_GROUP_PROFILE = False
 
+try:
+    from static_replace import replace_static_urls
+    HAS_STATIC_REPLACE = True
+except ImportError:
+    HAS_STATIC_REPLACE = False
+
 
 class ResourceMixin(XBlockWithSettingsMixin, ThemableXBlockMixin):
     loader = ResourceLoader(__name__)
@@ -222,6 +230,30 @@ class PollBase(XBlock, ResourceMixin, PublishEventMixin):
             result['errors'].append("Private results may not be False when Maximum Submissions is not 1.")
         return max_submissions
 
+    @classmethod
+    def static_replace_json_handler(cls, func):
+        """A JSON handler that replace all static pseudo-URLs by the actual paths.
+
+        The object returned by func is JSON-serialised, and the resulting string is passed to
+        replace_static_urls() to perform regex-based URL replacing.
+
+        We would prefer to explicitly call an API function on single image URLs, but such a function
+        is not exposed by the LMS API, so we have to fall back to this slightly hacky implementation.
+        """
+
+        @cls.json_handler
+        @functools.wraps(func)
+        def wrapper(self, request_json, suffix=''):
+            response = json.dumps(func(self, request_json, suffix))
+            response = replace_static_urls(response, course_id=self.runtime.course_id)
+            return Response(response, content_type='application/json')
+
+        if HAS_STATIC_REPLACE:
+            # Only use URL translation if it is available
+            return wrapper
+        # Otherwise fall back to a standard JSON handler
+        return cls.json_handler(func)
+
 
 class PollBlock(PollBase):
     """
@@ -387,7 +419,7 @@ class PollBlock(PollBase):
             ],
         }
 
-    @XBlock.json_handler
+    @PollBase.static_replace_json_handler
     def get_results(self, data, suffix=''):
         if self.private_results and not self.can_view_private_results():
             detail, total = {}, None
@@ -714,7 +746,7 @@ class SurveyBlock(PollBase):
                 return None
         return self.choices
 
-    @XBlock.json_handler
+    @PollBase.static_replace_json_handler
     def get_results(self, data, suffix=''):
         if self.private_results and not self.can_view_private_results():
             detail, total = {}, None