Fixed #06783: GCI: Deprecated <center> tag used to align button on print answers page

I searched for the &lt;center&gt; tag all over the PHP code where it was still present in only 5 places.

The center tag was replaced by a div having a class that does currently only the centering in the CSS.
I created 4 distinct classes since I cannot predict how the styling would evolve in each specific case.

I did not change the pstpl files in the templates basic, bluengrey and clear_logo.

A similar clean up for the &lt;font&gt; tag would lead to 101 files being altered. I did not take that huge a risk!