Fix two crashes when scanning QR codes
One crash was two due the `Context` being `null` and the other due to the camera being opened within an `AsyncTask`.
Closes#583
See merge request !282
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
