DUA form cannot be submitted with an empty "access_template" field

[tompollard]

The DUA form at https://localhost:8000/console/duas/1/ allows admin users to update Data Use Agreements.

The form includes an "Access template" field (corresponding to the DUA.access_template attribute). This attribute is not documented in the website or in the code, so its purpose is unclear.

It is not possible to submit the form when the field is empty. We should: 1) improve documentation so users understand what should be added to the field. 2) (perhaps) make the field optional, so the form can be submitted without the need to add content to the field.

[tompollard]

It looks like the field is used to create a "data_use_purpose" template in a form that is used by people to submit data access requests for "contributor-managed" projects.

https://github.com/MIT-LCP/physionet-build/blob/833a12485f439aad31896f866f1707e687dc62ad/physionet-django/project/forms.py#L961-L997