Allow changing, or adding, thumbnail for book #82
Description
We should probably also download and store the images lest they may go missing on the internet, or happen to be a cached image.
Suggested flow:
- click the image to get a URL-prompt
- if thumbnail is stored, delete it
- download the selected image and store it
This would mean that the image is always fetched from storage, but since the "no image" is already there that should not be a big problem.