Move Translation class inside Budget

The reason to move Translation class is explained here [1]

[1] 106649a8a5

app/models/budget.rb

@@ -229,4 +229,16 @@ class Budget < ApplicationRecord
     slug.nil? || drafting?
   end
 
+  class Translation < Globalize::ActiveRecord::Translation
+    validate :name_uniqueness_by_budget
+
+    def name_uniqueness_by_budget
+      if Budget.joins(:translations)
+               .where(name: name)
+               .where.not("budget_translations.budget_id": budget_id).any?
+        errors.add(:name, I18n.t("errors.messages.taken"))
+      end
+    end
+  end
+
 end