Make supporter_my_donate_page object available in $form_state

Problem/Motivation

When the submit handler salsa_my_donate_page_customize_form_submit() is executed during a creation of a supporter_my_donate_page, the supporter_my_donate_page object is created and saved.

It is not added to the form_state array though, which would be very useful in case you need to use it in a custom submit handler for example.

Proposed resolution

Add the created object to $form_state after saving it.