Add admin setting for default scope

What does this MR do and why?

This change adds a new application setting called "default search scope" that allows administrators to set a default category for global searches (like searching for issues, projects, users, etc.).

The feature includes validation to ensure that only valid search categories can be selected and that the chosen category is actually enabled in the system settings. For example, if searching for "issues" is disabled globally, then "issues" cannot be set as the default search scope.

The implementation adds the new setting in backend, creates proper validation rules, and includes comprehensive tests to verify that the validation works correctly for both the basic version and enterprise edition features. The setting defaults to an empty string, meaning no default scope is initially set.

References

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Before	After

How to set up and validate locally

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Related to #567844