Add tablet layout.

To do that, I had to remove all special activites for Feed/Article. Now
the content_frame is no longer flickering when navigating through feeds
and articles.

I still want to keep the drawer on tablets, but I have to move the
fragments into totally different positions and can't reuse the existing
IDs. I wonder if I have to write a new TabletMainActivity or if I have
to use lots and lots of IF-ELSE to differentiate the positioning of the
fragments for small and large screens.