[#607] Allow to reactivate deleted datasets

Currently if a dataset is deleted and users navigate to the edit form,
there is no state field and the delete button is still shown.

After this change, the state dropdown is shown if the dataset state is
not active, and the delete button is not shown.
Merge pull request #607 from okfn/607-allow-reactivating-datasets

Can not reactivate a deleted dataset from the UI