For #5885
Adds to, uses and cleans-up central permission checking in ImageService
to mirror that which would be experienced by users in the UI to result
in the same image access conditions.
Adds testing to cover.
Recent changes could mean missed references for images in non-page
locations. This fixes that, and tries to ensure images are used if we
already have a page-based image as part of the ZIP, otherwise ensure we
have a page as part of the export to attach the image to.
- Handled links to within-zip page images found in chapter/book
descriptions; Added test to cover.
- Fixed session showing unrelated success on failed import.
Tested import file-create undo on failure as part of this testing.
