Adjust front page layout when left/right boxes are not used

[osma]

At which URL did you encounter the problem?

http://finto.fi
http://skosmos.dev.finto.fi
http://browser.agrisemantics.org/en/
http://nfpa.firebrary.com/en/

What steps will reproduce the problem?

1. Look at front page

What is the expected output? What do you see instead?

The front page looks bad if no boxes have been configured. The boxes do not align in any reasonable way. We should make them line up better.

What browser did you use? (eg. Firefox, Chrome, Safari, Internet explorer)

Firefox

[osma]

Easy part of #412.