Wicket Site Improvements

[Tobias Soloschenko <to...@googlemail.com>]

Hi all,

I made some improvements to our site http://wicket.apache.org/

* Added content about Wicket 8
* Fixed some layout issues that a white space has been shown on the 
right side at mobile devices (iOS 9.x)
* Rearranged the button bars (3 max in one row), because some buttons 
got cut of at mobile devices
* Centered the mainmenu / submenu bars

Note that some links are not working currently, because the build fails 
during the decision whether to use Servlet API 3.1 or not. If the build 
is fixed all links are going to work.

We also should update the information about the Servlet API 3.1 in the 
Wicket-8.x download site after that.

kind regards

Tobias