added comparison "no cover" image to check against
added missing cover image exception
added compare_images method
set_cover_image_from_url is now able to check against a comparison image before setting
the importer now compares images before setting them on the issue
added basic placeholder "no cover image" image
updated templates to use "no cover image" image when cover is missing.
added "remove_cover_image" and "check_cover_image" methods
added "CleanCoverImages" management command - currently DOES NOT remove images - only checks for them
added processing text readout
added total issues to process and a notification when done.
added total image removed count
remove_cover_image both removes entry from database as well as removes physical file.
CleanCoverImages command now actually removes cover images.
added 'COMPARISON_IMAGE' and 'COMPARISON_IMAGE_URL' to general settings
comparison image is no longer hard coded into the importer
Merge branch 'feature/noimage' into develop

TFAW default cover images can now be detected using a hash. Command to remove
these images from the back catalog has been added as well.