Make the receipt_id custom field in receipt items an EntityRef field

[jensschuppe]

Follow-up to #172 - for full compatibility with SearchKit, this field referencing a receipt from a receipt item should become an EntityRef field for implicit FK constraints and JOINs.

This is not actually necessary for adjusting Donation Receipt views to SearchKit, rather a nice-to-have.

Notice by @colemanw:

Should be simple enough - just one word of caution. EntityRef fields get a real foreign-key-constraint in the database. So they must contain only valid foreign key references. Since the field currently has no constraint, it could be corrupted as easily as the referenced entity being deleted.