This ticket depends on #409 (closed) and #413 (closed).

When a blog from Available Blogs (#413 (closed)) has been clicked, it will be added to the "Blog List" tab. Here all subscribed blogs should be shown.
Phase 1

Order:
(1) unread messages with latest posts first (2) date of last post

Phase2
Implement possibility to pin an element to the top

Order:
(1) pinned blogs (2) unread messages with latest posts first (3) date of last post