Add FinacialTransaction create_collection for ordergroup custom fields

Custom fields of an ordergroup can set financial_transaction_source to
true, to act as an source for a new collection of FinacialTransaction.

A typical usecase would be a variable membership fee, which will be stored
in a custom field on the ordergroup. When a new membership period begins
a collection with all membership fees can be created with one click.

app/controllers/finance/financial_transactions_controller.rb

@@ -58,6 +58,14 @@ class Finance::FinancialTransactionsController < ApplicationController
   end
 
   def new_collection
+    @ordergroups = {}
+    Ordergroup.undeleted.order(:name).map do |ordergroup|
+      obj = { name: ordergroup.name }
+      Ordergroup.custom_fields.each do |field|
+        obj[field[:name]] = ordergroup.settings.custom_fields[field[:name]]
+      end
+      @ordergroups[ordergroup.id] = obj
+    end
   end
 
   def create_collection