Added config_updated_at attribute to User model.

It defaults to the current datetime.
Update the config_updated_at attribute of User instances when necessary.
Minor change in tests.
Added HTTP caching support to UserConfigsController#show action
Do not send a parameter with the current time when requesting the user config via Ajax.

It was keeping clients from leveraging HTTP caching.
Fix bug: update User instance from RefreshFeedJobState AR callback only if user exists.

When RefreshFeedWorker detects that the passed user does not exist, it deletes the RefreshFeedJobState from the database before exiting. This invokes the after_destroy callback of the RefreshFeedJobState model, which tries to update the refresh_feed_jobs_updated_at attribute of the associated User.

The bug was that, in this particular case, the User instance does not exist (the user_id attribute is set to null), which raised an error (because the update method was invoked on a nil object). The fix is, in the callback, to check that the User instance exists before attempting to update it.
Similar fix to f5ced1793cbec0cc61d814d1058a2d0da7961960 but for SubscribeUserWorker.

Also added test specifically for this behavior.
Similar fix to f5ced1793cbec0cc61d814d1058a2d0da7961960 but for FeedSubscription model.
Similar fix to f5ced1793cbec0cc61d814d1058a2d0da7961960 but for Folder model.