Subscribe to status updates

[leite08]

Is your feature request related to a problem? Please describe.

1. I want to allow users to subscribe to status updates from our service.
2. I want to subscribe to Checkly's status so I know when the service is down or degraded, because:
   1. This might impact how our customers see the status or our service;
   2. We want to know that our services might not be monitored for the duration of the inciden.

Describe the solution you'd like
An option on Checkly's status page allowing users to subscribe to updates.

Ideally, they would be able to choose between multiple ways to get notifications, like RSS/Atom feeds, email, Slack, maybe even webhook - see example from Github's status page:

Which is similar to Sentry's - it seems they use Atlassian Status Page.

I see #297 asks for a specific type of update, but I'd like to have more options than just email.

[ianaya89]

Related to #297