Item12207: Finish off user delete feature

Add a message if an invalid topic prefix is used.
Add ability to specify the prefix for deleted users
More unit tests

This will close the task for the feature.  There is one more enhancement
task: Item12699: Removing a user should clear any sessions for that
user.

git-svn-id: http://svn.foswiki.org/trunk/core@17200 0b4bb1d4-4e5a-0410-9cc4-b2b747904278