Blog Fragment in Navigation Drawer with Tabs

This replaces the custom layouts in the navigation drawer with a NavigationView and adds a menu entry for Blogs.

A Main Blogs fragment is added that holds a TabLayout and a ViewPager. Five tabs are already added, but they just have a single placeholder fragment that is to be replaced by the actual fragments.

Please note that the header in the navigation drawer is just provisional. I am sure @Megalox can design a better one.

Closes #409 (closed)