Decouple user personal namespace and profile

The user personal namespace and the user's profile are often used interchangeably. They both share the same URL path, however the two features serve different purposes. This distinction becomes important now that we're moving into the Organization/cell paradigm.

The personal namespace is a sandbox for user's projects. This sandbox is cell local.

The user profile is the public representation of the user to other users. The profile is cluster wide.

We need to split these two features. My recommendation is:

• Convert personal namespace to Group (&8757).
  • Disable personal namespace feature.
  • Convert personal namespaces to be groups.
  • Remove personal namespace feature.
• Make a User Routable.
  • Insert source_type=User routes rows.
  • Modify code base to reference the user profile where appropriate, instead of the legacy user.namespace association.

We need to sequence the work to avoid service interruption.