Drush migration command for removing distribution references

[dafeder]

Re-saves all datasets with un-referenced distributions (or any referenced property).

QA steps

w/orphans

1. Build site and import sample content
2. Run drush dkan:metastore:unreference-datasets (or drush dkan:unref)
3. Output should go through each dataset and show it is un-referencing distributions
4. Go to /admin/dkan/properties and confirm "distribution" is no longer checked in the "Dataset properties to be stored as separate entities" list
5. Go to /admin/dkan/datasets. Filter for "distributions". Ensure all are set to "orphaned"
6. Filter again for "datasets". Load one or two and confirm the distributions still appear (ie, there are download links).
7. Visit a datastore endpoint (e.g. /api/1/datastore/query/cedcd327-4e5d-43f9-8eb1-c11850fa7c55/0) and confirm data still loads

w/delete

1. Build site and import sample content
2. Run drush dkan:metastore:unreference-datasets --delete (or drush dkan:unref --delete)
3. Go to /admin/dkan/datasets. Filter for "distributions". Ensure list is empty.
4. Repeat 6-7 from above.