Namespace the Organization model

We should namespace the Organization model.

Ideally the namespace will be a namespace that encompasses the following models:

• Organization
• Project
• Group
• Namespace

The following discussion from !120118 (merged) should be addressed:

• @terrichu started a discussion: (+7 comments)

  question (non-blocking): instead of disabling this, should it be added to the exclude? .rubocop_todo/gitlab/namespaced_class.yml

/cc @alexpooley @rutgerwessels