HTTP caching in FeedsController#show uses only last_modified field, not etag.

In our case the app is more time-based than entity-based. Using last_modified exclusively makes more sense.
Removed extra parameter when requesting a single feed data via Ajax. HTTP caching now works as expected for requesting single feeds.
Reorganization of FeedsController#index. Action has been broken up into several methods, instead of a monolythic one.
Added subscriptions_updated_at attribute to Users model.

The idea is that this attribute will be updated every time there is a change in users subscriptions; that is, a change that would change the JSON returned by FeedsController#index. This way, HTTP caching can be implemented in FeedsController#index by using this new attribute as last_modified value.

Implemented: added attribute. It takes a default value of the current date/time when a User is created.
Not implemented: actually update the attribute when subscriptions are updated.