Statuspage Opt-in/Out

### Problem Statement

You often need subscribed to Statuspage (status.sentry.io) alerts before an incident happens, but only realize it even exists when there is an outage.

### Solution Brainstorm

We should provide a way to subscribe new users to Statuspage alerts (via an opt-in/opt-out mechanism on register, on SSO, etc). This ideally would also allow you to subscribe/unsubscribe from your notification settings similar to how we've done with marketing. That might mean doing an async lookup to SP (or similar).

I think at the very least, new account => option to subscribe, maybe checked by default, and then resolve the SSO creation page (probably similar to our terms acceptance prompt).

The one caveat here, is ideally we could focus on subscriptions to critical notifications. While you may want to know Jira is broken, you _need_ to know Sentry's entire service is broken. If theres an obvious solution to balance the noise we should take it.