Include a backslash character in wpEditToken to prevent editing from broken

Include a backslash character in wpEditToken to prevent editing from broken
proxies that mangle such characters.  Not only is editing from open proxies
generally frowned upon on Wikimedia projects, but more importantly, these
particular proxies tend to break wiki markup in submitted content.  These
really are not edits we want to let through.

Yes, this is a hack.  We may eventually want to explicitly check for this
condition and provide a more informative response, but even so this would
still remain a useful fallback check just in case.