Remove URLs from task and artifact serializers

Back before we thought of Ponos as an independent project that could be integrated into other apps, we thought that the Ponos server could be on a different domain name, or use a different API base URL, and therefore foreign keys could only be represented in the API using URLs. This is now irrelevant since Ponos is fully part of the Arkindex API, and since API clients do not support references to other API endpoints through URLs but only through operation IDs. We can remove the URLs and make sure we have UUIDs anywhere we need them to make the Ponos APIs consistent with the rest of the Arkindex API.

!2008 (merged) has removed the URLs to workflows by just removing workflows, but some URLs still remain on task serializers and are still used by the frontend and by the CLI's arkindex process commands. Building URLs in the frontend would be simpler since that's what we do for every other API, and the CLI can just use cli.request('RetrieveTaskFromAgent', id=…).

URLs are also present on artifacts: the Ponos agent ignores them and builds its own artifact download URLs, and the frontend uses them but could easily generate them too.