Support toggling dataset element uniqueness
https://redmine.teklia.com/issues/6264
Requires backend#1713 (closed)
The dataset creation and edition modals should allow toggling the new unique_elements
boolean field, with a checkbox. The checkbox is checked by default when creating.
The HTTP 400 error from PartialUpdateDataset
when enabling uniqueness on a dataset with duplicate elements, and the one from CreateDatasetElement
when adding a duplicate to a dataset, must both be handled.