Created a new test support macro to automate removing a feed from its current folder during acceptance testing. Modified existing tests to take advantage of this macro.
When the user removes a feed from its current folder, the unread entry count in its old folder is updated correctly (unless the folder is now empty, in which case it is removed both from the sidebar and from the Folder dropdown).

This has required changing the FoldersController#remove method, as well as the auxiliary FolderFeedRemove class:

- FolderFeedRemove.remove_feed_from_folder now returns the old folder instance, instead of a boolean. Whether the folder has been deleted or not can be later determined examining this object.

- FoldersController#remove now returns JSON with an HTTP 200 (OK) status if successful, instead of just a 204/205 HTTP status. It is necessary that the response is a JSON document that not only indicates if the old folder has been deleted, but also returns an HTML fragment to update the folder unread entries count, if the folder has not been deleted.