Update discussion counts on archive

For issue #1494 

I didn't see any attempt at counting motions per group other than a DB column, so left that for another pull request.

Is there a concept of unarchiving? If so we'll want to increment the counter if a discussion comes back to life somehow.
I think you can avoid this stuff. Rails provides methods for it. 
Just add counter_cache: true to the belongs_to :discussion association