Prevent asset validation if key tokens have no default values

[Grégoire_Miche2]

In order to avoid standard user errors, the admin should be able to flag (in the admin -> Field Management) which lead and account fields could not be used as tokens without a default value. Program tokens cannot be empty, so they are not in the scope of this.

When a field is flagged, if an asset uses the corresponding token without a default, the asset could not be approved.

-Greg

[Grégoire_Miche2]

Goes together with :

For maximum usability.

-Greg