Support registration of sites with sub-sub domains

[westi]

A correctly configured subdomain install can also host sub-subdomains.

At the moment we stop people registering them in wpmu_validate_blog_signup by not allowing a dot in the name.

A super admin can edit the site and change it though so they do work.

This should probably be configurable using a filter and default to off.

[ocean90]

Related: #20019

[nacin]

Since there is discussion on #20019.