Create an admin view for all currently active runs in a server instance

[sschuberth]

As an admin for a whole instance, I'd like to quickly get an overview over the runs that are currently active in any organization on that instance. It's basically a runs overview page that is not limited to a single repository.

Similarly, probably such overview pages would also be useful on product and organization levels, for admins for those levels respectively.

The main use-case is not to estimate the load on the server, but to get an overview when multiple runs triggered in different organization / products are finished.

[sschuberth]

As a note, such overview pages should probably be filterable by the user who triggered the run.