Adding the shell of a cookie check.  Cleaning up network.js, changing network.js's callbacks to be simpler.

* Adding complete to helpers.js, which required a reordering of scripts.
* Using complete in network.js so that existence of callbacks is checked automatically.
* Add a cookie_check.js module which checks which shows an error message if the user's cookies are disabled.
* Get rid of yesterday's code which triggered an event if cookies were disabled.

issue #835.