Add a warning when deleting an user from a group

Steps to follow to reproduce:

• Hover over "XWiki"; Click "Administer Wiki"
• Click on "Groups"
• Click on the pen to manager a group
• Add a user to the group (this way you won't later delete an important user)
• Click "Remove"

Result: the user is deleted without any warning message being prompted
E.g. why this may be a problem: An administrator could delete himself from the XWikiAdminGroup

A message should appear: "The User X will be deleted. Are you sure you want to proceed?"