Creative Commons license XMP metadata helpers

[trevorld]

• [x] xmpRights:Marked
• [x] xmpRights:WebStatement
• [x] xmpRights:UsageTerms
• [x] #42
• [x] cc:license
• [x] cc:morePermissions
• [x] cc:attributionURL
• [x] cc:attributionName
• [x] #40
• [x] #43

Some potentially useful links:

• https://wiki.creativecommons.org/wiki/XMP
• https://exiftool.org/TagNames/XMP.html#cc
• https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata#web-statement-of-rights
• https://github.com/adobe/xmp-docs/tree/master/XMPNamespaces

[trevorld]

• Creative Commons page recommends/supposes a "verifying" web statement
• IPTC/Google Images seem to suppose a web statement of license terms that is not verifying
• Creative Commons suggests dc:rights and xmpRights:UsageTerms would have same content. IPTC/Google Images supposes dc:rights contains copyright holder information whereas xmpRights:UsageTerms contains license information.