Add "Zero" specification to mwe-upwiz-source-ownwork-assert-cc-zero

Requested by Dcoetzee:
https://commons.wikimedia.org/w/index.php?title=Commons_talk%3AUpload_Wizard&diff=103561847&oldid=103045418

The legal code only says CC0:
https://creativecommons.org/publicdomain/zero/1.0/legalcode
The deed, CC0 1.0 Universal (CC0 1.0) Public Domain Dedication:
https://creativecommons.org/publicdomain/zero/1.0/

It makes sense to spell CC0 out for consistency, as the name
"Public Domain Dedication" alone refers to another tool as well.

Change-Id: Ic6ad0b57a2f3a93ff2f64c410ae9434961ab1cb8
Merge "Add "Zero" specification to mwe-upwiz-source-ownwork-assert-cc-zero"