Mass rollback utility

For recovering from vandalism, we should provide a mass rollback page in the Admin space, that can rollback all changes made by a specific user in a given timeframe.

• How do we find these pages? We can only search for documents not re-edited given the current way versions are stored.
• What if the document was re-edited? We should make a separate list with this kind of documents, for the admin to manually check.
• What do we do with this version? It's not OK to keep in the document history links to pill sites, adult sites and other kind of junk. We should have a means to completely delete document versions. (see XWIKI-962 )