Make POST_hide only allow Links through VByName.

Someone has been sending Comment fullnames to POST_hide and that
results in an error on our side and a 500 for them. They really
should be getting a proper error message back instead.

r2/r2/controllers/api.py

@@ -1725,7 +1725,7 @@ def POST_read_message(self, things):
 
     @noresponse(VUser(),
                 VModhash(),
-                thing = VByName('id'))
+                thing = VByName('id', thing_cls=Link))
     @api_doc(api_section.links_and_comments)
     def POST_hide(self, thing):
         if not thing: return