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.
NB: I tested this on my local environment, which only has a few manually created groups / discussions / motions. 

It worked fine there, but probably makes sense to run this through its paces in a staging / mocked prod environment before we loose it into the big bad world; there may be some cases I'm missing.