WIP: make only unread feeds and folders appear by default in the sidebar, and toggle them with a navbar button.

Since the recent change in which only unread feeds are returned by FeedsController#index, the client cannot know if a folder is to be totally removed from the angularjs model after removing a feed from the folder. It could be that the client removes a feed from a folder and there are no more feeds in the folder as far as the client knows, but in the database there are actually other feeds in the folder; it just happens that those feeds currently have no unread entries and therefore they have not been sent to the client. In this case the folder should not be visible in the sidebar, but it should be in the dropdown menu because it actually exists and the user can move feeds to it.

So, the client has enough information to decide that a folder should no longer be visible in the sidebar (if there are no feeds with unread entries in it) but it no longer has the necessary information to decide if a folder has to be removed from the model (which would make it disappear from the sidebar and from the dropdown menu).

Instead of implementing some way for the client to have the information to make such a decision, I've opted for a simpler strategy: after any action which could potentially cause a folder deletion, the client loads the folder list again and replaces it in the scope. This way any deleted folders will disappear both from the sidebar and the dropdown.

This strategy implies some overhead because the full folders list is sent each time, which is somewhat redundant. However I think moving feeds and folders around is not an action I expect users to perform often, and the simplicity in the code trumps this overhead.

In the case of feeds which still exist but should not appear in the sidebar, this is already accomplished with an angularjs filter.

This change has forced me to abandon the bootstrap-accordion behavior of folders in the sidebar, because the folders list replacement made it act funny. Now I'm just toggling the "in" class, which opens/closes the folder, based on which folder is marked as open in the global scope. Basing comparisons on folder IDs and not in object equality, this works fine through a total replacement of the folders list in the scope. Really, the accordion animation was pretty but not very useful; also I intend to remove it from the entries list to implement some autoscrolling, and it would be weird having the animation on the sidebar but not on the entries list. Not a huge loss.

Wow. That was the longest commit message for some time. If you've read to this point, I'm sorry for the rambling. All this behavior is a bit tangled in my mind.