holdingpen: content changed in the detail page can lead to invalid data

[ammirate]

The detail page in the holdingpen allows to edit some content, but no validation is performed. This leads to accept content that is not valid, making the current workflow fail.

Expected Behavior

Where possible, some cleanup is performed before saving the invalid data.

Current Behavior

Data is accepted as it is and record creation fails due to invalid content.

Steps to Reproduce (for bugs)

1. Submit a new article
2. Go in the holdinpen, to the halted submission
3. Remove all items from "Subject area"
4. Accept the article