sources: don't escape Unicode characters in tag search URLs.

Fix it so that Unicode characters aren't unnecessarily percent-encoded when generating tag search
URLs. For example, generate URLs like this:

* https://www.pixiv.net/tags/オリジナル/artworks

Not like this:

* https://www.pixiv.net/tags/%E3%82%AA%E3%83%AA%E3%82%B8%E3%83%8A%E3%83%AB/artworks

app/logical/source/extractor/nico_seiga.rb

@@ -77,12 +77,8 @@ module Source
       def tags
         return [] if api_client.blank?
 
-        base_url = "https://seiga.nicovideo.jp/"
-        base_url += "manga/" if manga_id.present?
-        base_url += "tag/"
-
         api_client.tags.map do |name|
-          [name, base_url + CGI.escape(name)]
+          [name, "https://seiga.nicovideo.jp/#{"manga/" if manga_id}tag/#{Danbooru::URL.escape(name)}"]
         end
       end