Add Managed Settings in GitLab

What does this MR do and why?

This adds the concept of Managed Settings.

Support for configuring a managed setting is dependent on support in Helm Charts. When a managed setting is configured there, it overrides its database configured value in GitLab's application settings (Admin UI).

A managed setting can't be modified via the Admin UI or the Admin API, keeping the Helm Charts as the SSOT.

References

part of #582262

Screenshots or screen recordings

Before	After

How to set up and validate locally

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