[dashboard] Re-implement 'Cancel Downgrade'

[jankeromnes]

Left-over from #3550

When I downgrade from a paid plan to a lower paid plan, I see the bottom label Downgrade Scheduled below the plan I'm downgrading to, with a Cancel link that shows up when the downgrade is confirmed as scheduled (i.e. no longer pending).

However, clicking on Cancel doesn't actually do anything. It should:

• Show a confirmation modal, explaining that you're about to cancel the scheduled downgrade to stay on your current plan

• When you accept the prompt, it should cancel the scheduled downgrade

Workaround: You can already cancel any scheduled downgrade via Billing, but it's a bit less convenient.