When the user requests to be subscribed from a feed, a job is enqueued to perform the unsubscribing instead of performing it immediately.

This should improve responsivity as perceived by the user.
WIP: minor changes in the angularjs client to account for unsubscribing now being performed in a background job instead of immediately.

Not yet working: currently if the unsubscribed feed was the only feed in a folder, the folder is not immediately removed from the dropdown menu.
WIP: resourceful route to get a list of feeds in a folder.

The angularjs client will use it to check if a just unsubscribed feed was the only feed in its folder; in this case, the folder will be removed from the dropdown menu.

This is necessary because the actual unsubscribing no longer happens immediately but is rather done in a background job; therefore the folder will not be destroyed until the job is processed, which may take some time. Despite this we want to give the user the impression that the unsubscribing and the folder being destroyed happen immediately: fake it till you make it!