ONYX-1699: Allow EnrollmentID to be editable for volunteers

[ymarcon]

Jira issue originally created by user @ymarcon:

If this field is set to editable in participants attributes configuration and if the participant is not enrolled, add a text field for it in the participant reception page.

As reported by Nick:

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Instead the Northampton colleagues will process participants as 'volunteers'. We have configured their Onyx server to allow that, and it is working almost exactly as intended. However I do have one question - is it possible to manually set an EnrollmentID for patients who are volunteers not enrolled? We use EnrollmentID to represent the hospital number of the patient, and it would be nice to be consistent. However, because the patients are volunteers, even with EnrollmentID set to be editable it does not appear on the data entry form. I note that the Onyx wiki says that this attribute is "typically" set by the uploaded appointment list, which makes sense, but the use of "typically" rather than "always" suggests that it may be possible to over-rule that behaviour and set EnrollmentID manually.