EE: reduce merge conflicts in the User model

What does this MR do?

This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22697. It includes the CE changes, combined with various EE specific changes to reduce the amount of lines that may conflict in app/models/user.rb when merging CE changes into EE.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/release/framework/issues/28

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder