Watch whole GitHub projects for issues being labeled with a specific label

As a reminder @heurtemattes, here's my idea for watching projects instead of sync'ing issues by manually adding them to the configuration:

It would be great if the config.yml syntax could be extended to allow something like

projects:
- url: https://github.com/eclipse-apoapsis/ort-server
  label: occtet
- url: https://github.com/oss-review-toolkit/ort
  label: eu-project

which means that the "ort-server" and "ort" GitHub projects are watched for issues being labeled "occtet" or "eu-project", respectively, and then these issues are sync'ed without a dedicated entry under issues:. That would streamline issue sync'ing quite a bit for projects where defining labels is under our control.

Some attention probably needs to be payed for not sync'ing issues twice if they're both listed under issues: and labeled.