Update discussion counts on archive

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.
The 'and' is there so that if update_attribute fails somehow, we don't decrement the counter, and so that `archive!` still returns true/false based on whether the update succeeds as well.