Add NSFW filter to preview images

[yspacedev]

This does cause a performance hit, but with parallel processing, it's not much.

This has been tested on an earlier version of the code, but it should still work.

[yspacedev]

This will throw an error on the first generation after SD is started for whatever reason (has to do with NoneType and a function). After that, it works fine.

[yspacedev]

This happens during the first final image safety check. For whatever reason the preview images are checked with no errors.

[AUTOMATIC1111]

nsfw detection is in an extension now; if you want, you can copy it and add your improvements there

[jamesblackwell]

is there any way to get the nsfw extension working with the API?