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.
Closes#409
The new activity shows who you are sharing a forum with and who shares a
forum with you. It is accessible from the overflow menu when in a forum.
Closes#398