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.
This will be a bit of a dyno-muncher :)

I figured it was alright for a one-off run, but if we need to optimize it some we can.