Resolve TMSS-698

Closes TMSS-698

Code looks ok. Tested the url: http://127.0.0.1:8000/api/task_blueprint/1/?expand=subtasks,subtasks.output_dataproducts&fields=url,subtasks.url,subtasks.output_dataproducts.size and indeed this works.

The frontend can use this expand mechanism in general. The above url is just a concrete example. If and only if the exapandable urls are to slow, then we can make an optimized view.

merged

mentioned in commit 14f67abe