Add FinancialLinks

For now this is only usefull for plugins, since there is no UI.

db/migrate/20171002000000_create_financial_links.rb

@@ -0,0 +1,10 @@
+class CreateFinancialLinks < ActiveRecord::Migration
+  def change
+    create_table :financial_links do |t|
+      t.text :note
+    end
+
+    add_column :financial_transactions, :financial_link_id, :integer, index: true
+    add_column :invoices, :financial_link_id, :integer, index: true
+  end
+end