If the browsers back button was used and the last form action was POST, we will displ...

If the browsers back button was used and the last form action was POST, we will display a message with a warning.
Cause some actions will be done twice in this case, like moving/deleting objects... or creating a freeze

git-svn-id: https://oss.gonicus.de/repositories/gosa/trunk@5213 594d385d-05f5-0310-b6e9-bd551577e9d8