TNL-1943 Add context field to CommentThread

AUTHORS

@@ -25,3 +25,4 @@ Bill DeRusha <bill@edx.org>
 Brian Beggs <macdiesel@gmail.com>
 Clinton Blackburn <cblackburn@edx.org>
 Eugeny Kolpakov <eugeny.kolpakov@gmail.com>
+Ben McMorran <ben.mcmorran@gmail.com>

models/comment.rb

@@ -141,7 +141,14 @@ class Comment < Content
   end
 
   def context
-    self.comment_thread_id ? self.comment_thread.context : nil
+    if self.comment_thread_id
+      t = CommentThread.find self.comment_thread_id
+      if t
+        t.context
+      end
+    end
+  rescue Mongoid::Errors::DocumentNotFound
+    nil
   end
 
   def course_context?