Add support for default license

[kauffj]

We will be adding a default content license (draft attached). This license should be made the default if no other license is specified. We should also consider allowing unspecified as a license if specifically suggested.

Either the SDK (or more likely, lbry.com) should also be responsible for storing the canonical license.

LBRY Default Content License (5-19-20) (2).DOCX