Show list of Available Blogs

This ticket depends on #409 (closed) and #412 (closed). A child fragment should be added to the TabLayout that shows all blogs the user can still subscribe to.

Update: Tab indicator and tags
Sorting order (from top to bottom):
(1) new+contact
(2) new
(3) contact
(4) date of sharing

A blog is considered as new when it was added in the last 24h hours. The date in the mockup refers to the time the blog was added and not the time of the last post, because that information is not available.