models: refactor class methods into scopes.

app/models/wiki_page.rb

@@ -21,6 +21,8 @@ class WikiPage < ApplicationRecord
 
   api_attributes including: [:category_name]
 
+  scope :active, -> { where(is_deleted: false) }
+
   module SearchMethods
     def find_by_id_or_title(id)
       if id =~ /\A\d+\z/
@@ -34,10 +36,6 @@ class WikiPage < ApplicationRecord
       where(title: normalize_title(title))
     end
 
-    def active
-      where("is_deleted = false")
-    end
-
     def other_names_include(name)
       name = normalize_other_name(name)
       subquery = WikiPage.from("unnest(other_names) AS other_name").where_iequals("other_name", name)