XWiki.ClassSheet allows guests to bind in memory an XClass to an existing sheet

XWiki.ClassSheet provides a link to bind the class to a sheet if the sheet exists but it doesn't check if the current user has edit rights on the class. Follow this steps to reproduce:

• log in
• go to XWiki.TagClass
• click on "Create the document sheet"
• you should now see the "Bind the sheet to the class" link
• logout; the link is still visible
• click the link; you get a Velocity macro error saying "Access denied in edit mode on document xwiki:XWiki.TagClass" but if you reload the page you'll see that the sheet has been bound.

The class has been changed only in memory, so a server restart will show that the sheet is not bound to the class.