support for item licenses

[maul-esel]

trello card #14

• Support _and require_ specifying an item's license or several licenses, each of which can either be:

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• a standard enumeration value for a common license (e.g. GPLv2)
• a link to the full license text
• the full license text

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• Also write a permissive "stdlib license"
• and require stdlib items to have set it as one of their licenses.