follow-up for #5348

currently you can only select from pre-defined licenses - and only by specifying a numeric license id.

So worked needed to:

• implement the ability to create a custom license
• select from predefined licenses by slug or some other mnemonic add license slug #8597
• maybe make the field optional (for listed versions only?) and default to all rights reserved, or the previous license for subsequent versions Don't require license for subsequent listed versions submitted via addon submission api #8599