Autogenerated custom domains don't produce inconsistency

When creating custom domains, Render sometimes creates a second redirect
domain by either adding or removing a `www.` prefix. We were including
these generated domains in the state for `custom_domains`, which is
considered an inconsistency error.

To fix this, we've introduced a new `active_custom_domains` property
that includes all custom domains, and restrict `custom_domains` only to
user-provided domains.