Make beta status on http://beta.mathhub.info very visible

[kohlhase]

I sometimes get confused when I am testing. I could imagine

• a light red page background via CSS
• a big yellow "beta" sticker in the right corner
• a combination
• ... It would also be good to add a sentence to the first page that explains the beta status, something like

  
  This is the public beta version of [MathHub](http://mathhub.info). 
  It is for community testing the development version of MathHub 
  and giving a preview of coming features. 

This site tracks the devel branch whereas the stable http://mathhub.info tracks the master branch.

Excuse the Markdown notation.