Would leave imported content with inaccessible images in many cases (or
wrong references) although the drawing was still being uploaded &
related to the page.
Added test to cover.
For #5761
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.
Images were missing their extension after import since it was
(potentially) not part of the import data.
This adds validation via mime sniffing (to match normal image upload
checks) and also uses the same logic to sniff out a correct extension.
Added tests to cover.
Also fixed some existing tests around zip functionality.
Added file creation reverting and DB rollback on error.
Added error display on failed import.
Extracted likely shown import form/error text to translation files.
