Add optional content_license string field for item objects

[przemoc]

People usually pay more attention to code's license, but text's license is also important.
In open source circles typically used ones for blogs are some of CC licenses: CC BY(-NC)-SA, CC BY(-NC) or CC0.
It would be useful to be able to explicitly state content's license in a unified way.

content_license (optional, string) at item-level mentioning license of the item could solve this.
(In theory it could be top-level field, but it would be much less flexible then.)

For what values could be put there, I would suggest using existing good database of license identifiers that has been made for SPDX:
SPDX License List

Examples:

• CC-BY-SA-4.0
• CC0-1.0
• FreeBSD-DOC
• MIT