Domains: disable creation on subprojects

[humitos]

If you go to Settings -> Domain of a subproject you will be able to add a domain. However, when saving it fails with Action not allowed. The POST request returns a 401.

The backend code that does this is at https://github.com/readthedocs/readthedocs.org/blob/34ee160d399eb1717700d36784ac49133108e982/readthedocs/projects/views/private.py#L852

We should do something at the frontend to avoid this.