Update prices on stripe price update event

[elboletaire]

Right now we need to restart the server in order for the prices to be synced with stripe. This is OK for a first version, but we may face errors by users if we forget to restart the server.

Stripe allows to create a webhook that listens on price.updated (and maybe also plan.updated) changes, which should allow us to silently upgrade the prices, without requiring a server restart.

Webhook docs: https://docs.stripe.com/webhooks
Events list: https://docs.stripe.com/api/events/types#event_types-price.updated