Annotate (in .json and annex level) closed captioning license/redistributability

[yarikoptic]

Only for videos under permissive license!

API: https://developers.google.com/youtube/v3/docs/captions/download

[yarikoptic]

doing that using WiP https://github.com/con/annextube but TODO:

• check assignment of annex metadata and other information about licensing -- is it per CC or per overall video terms (we have that)