New API endpoints to manage Dataset Sets

see #1712 (closed)

New endpoints are introduced to manage a dataset's sets, CreateDatasetSet, [Partial]UpdateDatasetSet and DestroyDatasetSet, under /api/v1/dataset/<id>/sets/ and /api/v1/dataset/<id>/sets/<id>/. They take a name as their request body. They must return an HTTP 404 if the dataset is not on a readable corpus. They must return an HTTP 400 if the dataset is readable but the user does not have admin access to the corpus, or if the dataset is not open.

DestroyDatasetSet should also return an HTTP 400 if this is the last set of the dataset, or if the set is selected by a dataset process. It should destroy all of the related DatasetElements before destroying the set.