Implemented HTTP caching in the FeedsController#index action when listing feeds in a specific folder.

Also the case in which the folder passed as parameter in the request doesn't exist or is not owned by the user is better controlled (a log line is written where before just a 404 was returned).
When indexing feeds in a folder, do not pass a parameter with the current datetime in the Ajax request.

This was done in the past to keep angularjs from performing client-side caching, but it's no longer necessary and in facts prevents the client from using HTTP caching in this action.