Fix #3208: Fix translated tag suggestions for Pixiv.

* Only suggest the Danbooru tag with the same name if there is no
  matching wiki other name. Example: if we have the Pixiv tag `Fate` and
  the Danbooru tag `fate_(series)` with other name `fate`, suggest that,
  not the Danbooru tag `fate`.

* Don't suggest tags that are empty or whose wiki is deleted.

* Only split tags on "/" if there are no other matches, and only for Pixiv.

* For Pixiv, only include traditional media tags in tag list, not digital media (Photoshop, SAI).

* Add some tests.

app/logical/sources/strategies/base.rb

@@ -80,6 +80,23 @@ module Sources
         (@tags || []).uniq
       end
 
+      def translated_tags
+        translated_tags = tags.map(&:first).flat_map(&method(:translate_tag)).uniq.sort
+        translated_tags.map { |tag| [tag.name, tag.category] }
+      end
+
+      # Given a tag from the source site, should return an array of corresponding Danbooru tags.
+      def translate_tag(untranslated_tag)
+        translated_tags = Tag.where(name: WikiPage.active.other_names_equal([untranslated_tag]).uniq.select(:title))
+
+        if translated_tags.empty?
+          normalized_name = TagAlias.to_aliased([Tag.normalize_name(untranslated_tag)])
+          translated_tags = Tag.nonempty.where(name: normalized_name)
+        end
+
+        translated_tags
+      end
+
       # Should be set to a url for sites that prevent hotlinking, or left nil for sites that don't.
       def fake_referer
         nil