Allow customer to cancel their own subscriptions within a specified time period

The feature request at #3091587: Provide the ability to cancel an active subscription → only allows admins to cancel subscriptions. I have seen many people asked for allowing customers to cancel their own subscriptions. So I'm just going to go ahead and create a child issue for it.

I am also adding an additional requirement which is to allow the admin to specify how many days the customer is allowed to cancel their own subscriptions. Typically, there is a legal requirement to allow customers to cancel within 14 days and get a full refund. The number "14" should not be hard though. So create an admin config to specify what that number should be. To make this requirement more generic for the wider community, there should be an option to allow cancellations indefinitely (ie no timeframe specified). This can be the default setting unless the admin chooses to specify the number of days.

I am not sure how to go about issuing a full refund once the subscription is cancelled. Maybe this can be another admin setting? Something like "Do you want to issue a full refund if the customer cancels?" And if unchecked do nothing, just cancel and no refund.

User interface changes

1. Put a cancel button on the "Your Orders" page next to each order.

2. Put a cancel button on the order details page.

3. Do not show any cancel button if the specified number of days have already elapsed.

4. Create 2 admin config settings (one to specify how many number of days or indefinite and another setting about issuing refund or not)