Site alias settings should be available as a server setting

At least some of the site settings should be per server.

The "Domain used for automatic subdomain hosting" would appear to be most useful if we were able to set it ( or remove it) per server.

"Use redirects instead of aliases by default" might have differing needs for development or production servers. similar for "Generate www.domain.com alias automatically"

Note from the Aegir maintainers:

This issue overlaps with:

• #1681904: Ability to configure a url to redirect to in site configuration.
• #815190: Allow for both redirects and aliases
• #1245198: Site alias settings should be available as a server setting
• #1004526: Automatic aliases are not persisted across rename and clone