[BE] Cancellations

[Gaspard Boursin]

Add an api to cancel a subscription (with an optional cancellation reason)

Termination can be :

• at the end of the period
• now
• at a date in the future

Mark the subscription as cancelled with the proper cancellation date/end date.
Make sure no invoice is emitted past this point.

If now/specific date is NOT the end of the period :

• emit an invoice at cancellation if there are in-arrear fees
• if there are advance fees that have been paid for the full period, we will emit a credit note for the unused number of days, make it a placeholder for now
• check that if a period is partial because of the end date, we apply prorating too (for now we only checked for a partial first period)

[Artur Jurat]

for the start implemented with at the end of the period