Custom domain: don't allow external domain

[stsewd]

We have a CNAME from our external domain to readthedocs.io, making these domains valid if they are added to a project.

This isn't a security issue, since our code won't resolve those domains because custom domains are checked last, and we have this protection in place:

readthedocs.org/readthedocs/core/unresolver.py

Lines 590 to 594 in 7daff3d

	if public_domain in domain or external_domain in domain:
	# NOTE: This can catch some possibly valid domains (docs.readthedocs.io.com)
	# for example, but these might be phishing, so let's block them for now.
	log.debug("Weird variation of our domain.", domain=domain)
	raise SuspiciousHostnameError(domain=domain)