[#68811824] Add delete back into the admin UI

- Doesn't remove data-set from backdrop
- Admin UI automatically gives safety warning
Add in test for deleting data-sets

- Remove old test that asserted that we can't delete data-sets
- Mocked as per all the others tests
Remove DeleteNotImplementedError on delete method

- Also add in `purge`
Rename CREATE_COLLECTION_ENDPOINT_TOKEN

- Use STAGECRAFT_COLLECTION_ENDPOINT_TOKEN instead, as we want to use it for deleting collections as well
Test that caches are purged on delete
Remove unused DeleteNotImplementedError exception
Removing unnecessary test
Renaming functions for creating/deleting data-sets
Updating some of the DataSet model's docstrings
Test delete_data_set() from backdrop_client is called
Fix and refactor delete_data_set()
Call create_data_set() and the super method when deleting from the ORM
Merge pull request #119 from alphagov/delete_datasets

Delete datasets