Use svg images for the nsfw spoiler image

@shmibs has remade the nsfw spoiler images in svg so they can be themed to match everything else. Also now they don't look blurry on anything above 1080p. They can be found at https://tomo.airen-no-jikken.icu/objects/4c247daf-e08b-45f4-90ec-874ff0bf79b8

Here is how the nobg version looks with light and dark themes: