Ad groups and channels

I think most of this will be cleverly solved by ✨ Turn ad sizes into config entities Active .
We can afterwards discuss what else might be needed and someone who needs it can create a MR. Until that, let's set this issue POSTPONED on ✨ Turn ad sizes into config entities Active

@clau_bolson: Ad channels might still be an interesting feature for the future, while I think ad groups logic will be solved as written in #7.

If you're still interested, please create a feature-request issue for ad channels and describe how they should work. We'd be happy to review MR or do some sponsored development on that. I'll keep this as postponed for now.