fragment_subject: Replace the RelativeLayout with a LinearLayout.

It only has one child anyway.
Tablet layout: Classify: Split the screen equally horizontally.

With the image (SubjectFragment) on the left and the rest
(ClassifyFragment) on the right. To get this effect I specified
equal weights for each child view, but I also had to specify
width="match_parent" to make it actually work, which is odd.