Domain verification - Add domains

Release notes

Problem to solve

As described in &5299, group administrators need to be able to add domains to their group.

Proposal

• Enable group owners to add a domain to a top-level group.
• Owners should be able to add more than one domain. We can use the same front-end experience as https://docs.gitlab.com/ee/user/project/pages/custom_domains_ssl_tls_certification/index.html . NOTE: We don't need the certificate or key parts of this process.
• We should perform basic validations to ensure that the domain is valid.
• Once a domain is entered, we should generate and return a txt record.
• We'll tackle the process for domain verification and deletion in separate issues. This issue will be solely for capturing and storing the domain.

Further details

I'm including screenshots of the domain verification process for pages for reference.