media assets: fix tagged filenames option.

Fix the `enable_seo_post_urls` config option not being respected. This
option controls whether filenames in image URLs contain the tags. This
option requires URLs rewrites in Nginx to work so it's disabled by
default.

app/models/media_asset.rb

@@ -67,6 +67,7 @@ class MediaAsset < ApplicationRecord
         "/images/download-preview.png"
       else
         slug = "__#{slug}__" if slug.present?
+        slug = nil if !Danbooru.config.enable_seo_post_urls
         "/#{variant}/#{md5[0..1]}/#{md5[2..3]}/#{slug}#{file_name}"
       end
     end