Allow Metabase to be configured with IP whitelist

[remeika]

One of the major limitations of hosting applications on Heroku is that Heroku Router has no capability to whitelist IP addresses or do certificate pinning; instead, individual apps are responsible to enforce these restrictions in order to leverage security products like a WAF.

It would be amazing if the Heroku Buildpack could accept a list of IPs/CIDR masks in an environment variable, and then configure Jetty to only allow access from these IPs, based on the X-Forwarded-For header that is enforced by Heroku.

I know this might be a "core" Metabase request, but I added it to this repo because Elastic Beanstalk has reasonable security options available in the Application Load Balancer that would allow the operator to do this whitelisting before the request ever hits Jetty; the Heroku, the need for whitelisting is much much greater.