Revert "Merge branch 'fix_email_images' into 'master'"

This reverts commit d66148ef, reversing changes made to cdb64a81.

This change needed to be reverted, because not enough email clients support inline images.

/cc @DouweM As discussed before. I'll try to create a MR to allow access to all images this week. I'm a bit busy at the moment so no guarantees. ;)