WorkerVersions as sources are neither in the API nor the frontend

Sentry Issue: ARKINDEX-FRONTEND-CK

TypeError: e.source is null
  at call (./vue/Navigation/ChildrenTree/TreeItem.vue?536f:1:1171)
...
(18 additional frame(s) were not displayed)

backend#365 (closed), backend#366 (closed), backend#367 (closed), backend#368 (closed) all added new foreign keys to slowly migrate from DataSources to WorkerVersions, but those are not always returned in serializers and the frontend does not handle "source": null when something uses a WorkerVersion instead of a DataSource, causing those TypeErrors.

The frontend should be able to display the worker version for elements and transcriptions instead of the source (requires backend#388 (closed))