Upgrading a project from 6.0 to 8.10, Document.AddPermission loses username value

PROBLEM

• Take a Studio project targeting Nuxeo 6.0.
• Add a test automation chain and the operation Document.AddPermission with a permission ReadWrite and a username Administrator, and save the automation chain.
• Now, in application definition, target Nuxeo 8.10 and come back to your test automationchain: the username appears blank.
• Now, in application definition, target Nuxeo 6.0 and come back to your test automationchain: the username is still there.

Expected: username value is not lost in retargeting toward another Nuxeo version and value is shown.

TODO
Add a validation warning when:

• TP is > 6.0
• Value for user field is not empty
• Value for username field is empty

• Validation warning content:
  In operation Document.AddPermission, field user was renamed to username since Nuxeo Server LTS 2015. You might wish to fill in this field with the value [user field value] that you were using in Nuxeo Server 6.0.

• Make sure that the value shown in the validation warning is properly escaped to prevent XSS flaws.