WIP: Resolve "Allow global read-only user (like admin)"

What does this MR do?

• Enhances the management of Admins to offer a "Read Only" type role that allows a user full read only access to all resources.
• Removes the admin boolean from User.
• Adds a role_type enum on User that accepts Default, Admin, & Auditor
• Adds a filter in the admin interface for filtering users based on Auditor role.
• Policy changes to allow read only access to projects & related resources, and groups.
• Project and Group explore pages are expanded to show Auditors all available projects and groups.

Are there points in the code the reviewer needs to double check?

• Migrations should be double checked.
• Didn't find information about backwards compatibility requirements in the API. API may require review as changes have been made.

Why was this MR needed?

The compliance department of a customer wants to run tests against the entire GitLab base to ensure users are complying with password, credit card, and other sensitive data policies. Without giving them full admin privileges.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?