Annotate open-access links with specific licensing information

[leonardr]

Each open-access link we serve should have a <dc:rights> tag indicating the specific rights under which the book is being made available.

An OPDS importer should be able to pick up such a tag. It should probably also be able to pick up an <atom:rights> tag as explaining the rights on every acquisition method for a book.

Moved to core from https://github.com/NYPL-Simplified/content_server/issues/67.