Order translations using ruby

Joining the translations table caused duplicate records to appear.
Ordering with SQL is simply too hard because we need to consider
fallback locales.

Thanks Senén for providing most of the tests in the poll spec.

app/models/budget/heading.rb

@@ -40,8 +40,7 @@ class Budget
 
     delegate :budget, :budget_id, to: :group, allow_nil: true
 
-    scope :i18n,                  -> { joins(:translations) }
-    scope :allow_custom_content,  -> { i18n.where(allow_custom_content: true).order("budget_heading_translations.name") }
+    scope :allow_custom_content,  -> { where(allow_custom_content: true).sort_by(&:name) }
 
     def self.sort_by_name
       all.sort do |heading, other_heading|