- Updates book/shelf cover image handling for easier cloning/handling.
- Adds core logic for promoting books/chapters up a level.
- Enables usage of book/shelf cover image via API.
Related to #1087
For review of meta tag additions as per PR #2393.
This commit removes any image guesswork and only uses images that have
been set by the author for the specific content.
This also adds tests to cover the expected OG tags.