Pages: Add instance-level default unique domain setting

Summary

Currently, GitLab Pages has unique domain enabled by default, with users able to disable it at the project level. This issue requests adding an instance-level setting that allows administrators to control the default unique domain behaviour across the entire GitLab instance.

Current Behaviour

• Unique domain is enabled by default for all Pages projects
• Users can disable unique domain on a per-project basis
• No instance-level control over the default setting

Proposed Solution

Add an instance-level admin setting that controls the default unique domain behaviour under Admin > Settings > Preferences > Pages:

• Setting name: "Enable unique domains by default"
• Default value: Enabled (maintains current behaviour)
• Effect: When disabled, new projects will have unique domain disabled by default
• Scope: Affects default setting for new projects; existing projects maintain their current configuration

Expected Behaviour

1. Admin Configuration: Instance administrators can toggle the default unique domain setting in the admin area
2. Project Creation: New projects inherit the instance-level default setting
3. Project Override: Users can still override the default at the project level (enable/disable as needed)
4. Existing Projects: No change to existing project configurations