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.
Yeah, but the `motions_count` column is on Group, not Discussion, so putting it on the discussion association won't work. I googled around a bit about how to implement a has_many :through counter cache; seemed like create/destroy callbacks were the cleanest option.

We could move the column to Discussion, and just have group sum up the motions_count of its discussions, but it halfway defeats the purpose of having a counter cache to load up all the discussions to get the count.

I'm not totally clear on where all this stuff gets used, so maybe moving the column down to Discussion makes more sense anyway.