Generalising create/update form changes from SMS module

Much of the form code in SMS example can be moved into the general module. Notes:

To make prefill, get the field to fill:-

$type = $form['#bundle'];
$bundle = entity_load('subscriber_entity_bundle', array($type));
$bundle = reset($bundle);
$field_name = $bundle->unique_field;

Add a hook for people, or make it plugins, so they can send the correct 'revision verification' confirmation message.