Send email confirmation when purchasing shares

[auggod]

From polis Email confirmation for share purchasing would be quite helpful, especially as a new member. I paid for shares, dunno if I'm member yet.

https://community.resonate.coop/t/resonate-polis-1-a-poll-and-conversation-leading-into-the-annual-general-meeting/3357

Purchasing shares does not grant member status. This should be more clear. Related: #6

To send this email, we can add additional code here: https://github.com/resonatecoop/id/blob/develop/webhook/stripe_payment.go#L183

A template will need to be added to mailgun.