Allow sorting widget_cards on homepage

Note that we keep :created_at order as complement to new :order field.

We do this so that current installations will not notice any change in the
sorting of their cards when upgrading, as the default "order" field will always
be 1, so it will continue to sort by the "created_at".

app/models/widget/card.rb

@@ -12,12 +12,14 @@ class Widget::Card < ApplicationRecord
   validates :link_url, presence: true, if: -> { !header? || link_text.present? }
   validates :order, numericality: { greater_than_or_equal_to: 1 }
 
+  scope :sort_by_order, -> { order(:order, :created_at) }
+
   def self.header
     where(header: true)
   end
 
   def self.body
-    where(header: false, cardable_id: nil).order(:created_at)
+    where(header: false, cardable_id: nil).sort_by_order
   end
 
   def header_or_sdg_header?