Allow super admins to configure custom domains w/o eng intervention

[jnaviask]

Description

Currently, our growth team needs to ask whenever a custom domain is needed or changes. We want to remove the engineering overhead here by allowing them to do it themselves.

Engineering Requirements

Form will need to:

• Accept a custom domain from an external user.
• Submit the domain to heroku, and display the response as a heroku DNS target.
• Update the community's custom_domain field to the provided domain.

Acceptance Criteria

tk

Additional context

Previously we had an "Airplane" script which performed the task. We do not want to use Airplane, but it may be a useful reference for the steps involved.

We should also consider the architecture here so that when #4882 is implemented, we're ready to move the logic immediately.

[dillchen]

Requires Magic Login Custom domain allowlist as well