Add an endpoint to get currently active runs in server instance

[lamppu]

This would allow to get the data needed for the admin view described in #837.

[lamppu]

I could take this on, if it's ok. I wonder though if there will be a need to get runs by other statuses as well, so should this endpoint just be /runs, and it could be filtered with a query parameter like status=active for the use case in the UI issue.