New Social Domain type to contain Social Workspaces

When creating a domain in the Root document, show the standard popup
of document creation where we have the choice between Domain (the
standard one) and Social Domain.

In a SocialDomain, only SocialWorkspace can be
created.SocialWorkspace cannot be created outside of a SocialDomain.

Remove the creation of the Social Workspace container, and the
container (type, etc) itself.

In Social Collaboration, override the default content template to
create, with the default domain, a new SocialDomain named
"Collaboration"