Validate webhook using signature instead of re-fetching the event

[paltman]

Issue Summary

Stripe now provides webhook signatures so we can drop the old method of protecting against fake events posting to our endpoint by validating the event using the signature.

[tstirrat15]

@tysonclugg what's the status of your branch? Will it be going to PR soon? As an interested user, I'd love to see this in a release.

[tysonclugg]

@tstirrat15 I had some questions regarding updating the docs, which are partially answered. Aside from the docs, my branch is ready for review.
I've been snowed under at work and this got sidelined, but I may have some time today to finish the documentation updates and submit my branch for review.

[tstirrat15]

Cool. Let me know if I can help in any way.

[tstirrat15]

@tysonclugg any updates?

[tysonclugg]

@tstirrat15 I'll have a look on Monday.

[paltman]

Done in 0d6da79