Added unread_entries column to the Folder model (and associated database table). It will serve as a cache of the unread entries count in the folder, instead of having to calculate it on each page load.

This should speed up application loading.

The migration that adds the column also populates it with the current count.

Pending: update this column each time it is appropiate (a new unread entry is added to a feed in the folder, an entry changes state unread->read, etc).
WIP: automatically update each folder unread entries count when appropiate (entries added etc).