Introduce PersonalNamespace and make Namespace abstract

For now we're treating Namespace as a personal namespace, and Group a subclass of Namespace. This somehow introduced some confusion and it would be nice if we could have PersonalNamespace as a personal namespace, and Namespace goes abstract. The intention would be much more clear this way.

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The following discussion from gitlab-ce!18464 should be addressed:

• @godfat commented on a discussion: (+2 comments)

  I think PersonalNamespace is a good idea and might not be so hard to implement. Namespace is really confusing. Let's make an issue.