Fix migration of default quantities

Allow Quantity formula to be nil but not blank
Add computed energy quantity
Reverse default quantities deletion order

app/controllers/quantities_controller.rb

@@ -59,6 +59,7 @@ class QuantitiesController < ApplicationController
   private
 
   def quantity_params
+    params[:quantity].delete(:formula) if params[:quantity][:formula].blank?
     params.require(:quantity).permit(
       :domain,
       :parent_id,

app/models/quantity.rb

@@ -15,7 +15,7 @@ class Quantity < ActiveRecord::Base
   validate if: -> { parent.present? } do
     errors.add(:parent, :parent_domain_mismatch) unless domain == parent.domain
   end
-  validates :formula, formula: true
+  validates :formula, formula: {allow_nil: true}
 
   after_initialize do
     if new_record?