Create get_dependent_jobs API

[simonbray]

Implement an API for “what has been done with this dataset”. This is tracked but not exposed currently.

suggested by @bgruening here: https://github.com/usegalaxy-eu/issues/issues/265. It would need to be implemented in the API first, similar to the provenance API (https://github.com/galaxyproject/galaxy/blob/dev/lib/galaxy/webapps/galaxy/api/provenance.py) but using item.dependent_jobs instead of item.creating_job.