BUGFIX: Fixed bug where you couldn't edit templates that had no subdomain.
ENHANCEMENT: Mark global groups in the subsite list, rather than subsite groups.
ENHANCEMENT: When creating a subsite from a template, copy the groups as well as the pages, but leave the groups empty
ENHANCEMENT: Got 'Is Default Site' checkbox actually working; if used the main site option is removed from the sites list when editing content.
API CHANGE: Added $subsite->absoluteBaseURL(), as a subsite-specific equivaent of Director::absoluteBaseURL()
BUGFIX Added Subsite::allowed_domains() which selects base_domain() if no custom domains are specified. This avoids having an empty dropdown in the URL domain selection dropdown in SubsitesAdmin
