add a get_num_followers endpoint

api/comment_threads.rb

 get "#{APIPREFIX}/threads" do # retrieve threads by course
-  
+
   threads = Content.where({"_type" => "CommentThread", "course_id" => params["course_id"]})
   if params[:commentable_ids]
     threads = threads.in({"commentable_id" => params[:commentable_ids].split(",")})
@@ -105,3 +105,16 @@ get "#{APIPREFIX}/courses/*/stats"  do |course_id| # retrieve stats by course
     course_stats.to_json
   end
 end
+
+get "#{APIPREFIX}/threads/:thread_id/num_followers" do |thread_id|
+  begin
+    exclude_user_id = 0
+    if params.has_key?("exclude_user_id")
+      exclude_user_id = Integer(params["exclude_user_id"])
+    end
+    thread_followers = {
+      "num_followers" => Subscription.where(:subscriber_id.ne => exclude_user_id, source_id: thread_id).count()
+    }
+    thread_followers.to_json
+  end
+end