Add use_slave to urls that don't update the DB.

The views from django.contrib.syndication.views aren't standard Django class-based views so they can't be used with django_replicated.

Other views are a mix of updates and reads, so need to be updated within the view to use the replicant.
Merge pull request #1313 from LLK/feature/use_replicant

Add use_slave to urls that don't update the DB.