Add registrant verification versions table according to existing logic

Creates a log_registrant_verifications table and version class as per https://github.com/internetee/registry/issues/1425#issuecomment-567117002

db/migrate/20191227110904_add_json_based_version_to_registrant_verifications.rb

@@ -0,0 +1,24 @@
+class AddJsonBasedVersionToRegistrantVerifications < ActiveRecord::Migration[5.0]
+  def change
+    name = 'registrant_verification'
+    table_name = "log_#{name.tableize}"
+
+    create_table table_name do |t|
+      t.string   :item_type, null: false
+      t.integer  :item_id,   null: false
+      t.string   :event,     null: false
+      t.string   :whodunnit
+      t.json     :object
+      t.json     :object_changes
+      t.datetime :created_at
+      t.string :session
+    end
+    add_index table_name, [:item_type, :item_id]
+    add_index table_name, :whodunnit
+
+    add_column name.tableize, :creator_id_tmp, :integer
+    add_column name.tableize, :updater_id_tmp, :integer
+    rename_column name.tableize, :creator_id_tmp, :creator_id
+    rename_column name.tableize, :updater_id_tmp, :updater_id
+  end
+end