improve invoicing

Please consider updating the invoicing feature with below:

1. Seach invoice by purchase order number

2. Add $0 course fee to an invoice (we have additional free course but needs itemised in the invoice)

3. Import the course ID on the description on the invoice (there are invoice with multiple courses from same date and need to differentiate them)

4. Make ‘expected’ number of students to be able to stay at 0 or make it possible to change it after invoice has been created (we use this to track the participant enrolment for inhouse courses)

5. When price is updated on the invoice for inhouse course, update the price in course course page (where you have to ‘update’)

6. When ‘tentative’ booking’s invoice for public booking is cancelled, change the ‘required’ amount to $0