Draft: Organization read-only/maintenance mode POC

What does this MR do and why?

This is a proof-of-concept showing how it might work to implement a read-only or maintenance mode for Organizations. This follows a very similar model to our existing read only mode for the instance.

This implementation is largely written by Duo and was intended to be a quick discovery effort. We would take a lot more thought and care into how this would look in production and we would test for edge cases.

References

Implement Organizations Read-Only Mode (gitlab-org#20404)

Screenshots or screen recordings

How to set up and validate locally

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