Allow Access to Subdomain Yaml Property of Pages

Allow access to subdomain property of pages in the CMS/pages UI, so developers can build pages using this feature that are available outside of CLI.

Default user-role should be agency, not client.

Simplest thing would be to make it a free text field, but even better would be a multi-dropdown from options defined somewhere globally (may need to be a longer list than actual domains on the site).

https://documentation.platformos.com/developer-guide/pages/pages#subdomains