ヘルプを Wiki から分離 (#424) (#430)

画面上でのテストは完了．
移行にす早さが求められるので，準備ができたらマージする．

Reviewed-on: #430
Co-authored-by: miteruzo <miteruzo@naver.com>

backend/app/controllers/materials_controller.rb

@@ -70,7 +70,10 @@ class MaterialsController < ApplicationController
         .find_by(id: params[:id])
     return head :not_found unless material
 
-    wiki_page_body = material.tag&.tag_name('ja')&.wiki_page&.current_revision&.body
+    wiki_page_body =
+        TagDescription
+        .find_by(tag_id: material.tag_id, language_code: locale.language_code)
+        &.body
 
     render json: MaterialRepr.base(material, host: request.base_url,
                                    language_code: locale.language_code).merge(wiki_page_body:)