upgrade cas-server-webapp to latest version

This pull request upgrades cas-server-webapp to 4.0.0 release.

If a custom configuration has modifications to the cas UI then it is possible they will have to be slightly modified for this new version.  However, the UI is very similar and likely only need minor changes if any are required at all.
Out of curiosity, if the cas client 3.1.0 still compatible to check tickets against a 4.x version of CAS server ?