CachedMixin should only concern itself with cache functionality.

@tuxcanfly Removed extra_context handling functionality from CachedMixin. 
Now there is some duplication because both CachedTemplateView and CachedListView need to handle extra_context. It can be handled by creating another mixin ExtraContextMixin that handles extra_context. And CachedTemplateView and CachedListView can extend from ExtraContextMixin.