names: Allow multiple static domains to be configured

Currently, when configuring a regular static domain in 'config' app, we allow only one domain. Allowing, multiple domains could provide some minor advantages.

• If the user owns the domain and subdomains, each could then be configured for a different subdomain.
• User could use a single FreedomBox to host multiple people's domains (each one, say, based on their name).

All our apps already assume that multiple domains exists in the system due to various sources such as static domain, dynamic dns, tor hidden service and pagekite. So, from the perspective of apps, there would not be any change.