[SE-2869] Forbid registering Ocim custom domains that conflict with current or past clients' domains

Created by: fferrin

This PR prevents users from using invalid or conflicting domains

JIRA tickets: SE-2869

Testing instructions:

This PR already provides tests, but for manual testing you can try:

• Using opencraft.hosting as a domain
• Using as domain a subdomain from an instance. For example, using test.domain.com when domain.com already exists
• Using a domain such that, when creating studio, ecommerce or studio subdomains, it overrides an existing domain. For example, try using domain.com when studio.domain.com was used as a domain.

Reviewers

• @swalladge