* riece-300.el: Rewrite using riece-decode-identity.

* riece-300.el: Rewrite using riece-decode-identity.

* riece-identity.el (riece-completing-read-identity): Signal an
error when the encoded channel name is not matched with
riece-channel-regexp.

* riece-globals.el (riece-channel-regexp): Moved from
riece-channel.el.
(riece-user-regexp): Moved form riece-user.el.