Allow custom domains for Opsgenie instances

Today, you can sign up for certain Opsgenie under the subdomain of your choice. For example, if your company is Acme, you can host these products under acme.app.opsgenie.com 

The request for "custom domains" on this ticket is to allow customers to use Opsgenie on their own second-level domain, i.e. opsgenie.acme.com or acme.com/opsgenie