Multi-store reporting for server admin

After a lot of reporting & accounting improvements in 2.2.0, there's one more feature, often requested by people, and something we've seen as highly necessary at the conferences during merchant setups.

A server admin, should be able to see Reports on all servers on their stores. It indeed could be a privacy issue, but merchants are already trusting server operator with privacy and security by relaying on their node, so all of these information technically is already available to the server operator.

Since the entire UI is coupled to a Store concept (even dashboard is store-centric), the only way to expose this for the admin would be through Server settings.

Reporting for all stores imo should be the same as for single stores. The UX challenge here is that once again we'll be duplicating UI's (same like we did for users, roles) between Server and Store.

This issue isn't specified, and shouldn't be tackled until @NicolasDorier @dstrukt and myself specify it. Most likely it will be tackled by @NicolasDorier once specified.