Missing purchase ID for donations

idealdonation.module did not set a purchase ID for transactions, causing them to fail. For lack of db_next_id() in Drupal 6, a temporary fix has been committed, which uses the transaction description as the purchase ID as well.

We do need a unique purchase ID, however. A transaction object is created for every page the form is displayed at. If the form is displayed in a block, a new purchase ID will be generated on a lot of pages, causing those IDs to increase rapidly. We don't want to run out of purchase IDs, so we would need to set it in a custom submit handler for the transaction form, before the default submit handler is called.