OK, some massive changes with many files removed or changed.

OK, some massive changes with many files removed or changed.

Basically the changes are:

 - I've merged the 'discuss' module into the forum module
   which makes the interface MUCH clearer for everyone
 - I've added a new 'single' forum type that replicates
   what the old discuss course modules used to look like.
 - I've got rid of the "discussion" forum type - it will
   still exist in upgraded courses but as a normal forum.
 - the 'discuss' module is completely deleted - gone.
 - the 'chat' module is completely deleted - gone.
 - The upgrading system has been improved, and all code
   is stored in version.php.
 - I've put in upgrading commands to do the best I can
   (right now) to upgrade courses that used the discuss
   module.  It should mostly work, just leaving some
   "orphan" coursemodules on you course front page.  You
   can easily delete these using the little 'x'.
   I may have forgotten something  - I've only tested on
   my testing server and I'm about to test on my production
   server to see how it goes.
 - Forums have a lot of little new features and fixes.  The
   main one is the subscription process.  Teachers can 'force'
   subscriptions on any forum.  This disallows everyone from
   choosing their own mail subscription - it's just on.
 - The assignment module is half-finished and not working yet

I've still some massive changes to do, mostly involving making
all the lib.php function names more standardised, so consider
this is an interim checkin to do some tests.