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/models/tag.rb

@@ -787,6 +787,10 @@ class Tag < ApplicationRecord
   end
 
   module SearchMethods
+    def nonempty
+      where("tags.post_count > 0")
+    end
+
     def name_matches(name)
       where("tags.name LIKE ? ESCAPE E'\\\\'", name.mb_chars.downcase.to_escaped_for_sql_like)
     end