MCP: unhide mcp scope on admin OAuth applications form

Child of #595569 (closed). Phase 1 of the OAuth-app-reuse work.

Unhide the mcp (and mcp_orbit) scope checkboxes on /admin/applications/new so admins can pre-register MCP OAuth applications.

• Add include_mcp_scopes: to OauthApplications#load_scopes.
• Wrap in a load_admin_scopes helper inside Admin::ApplicationsController.
• Specs: admin form renders mcp; user-level form does not.

See #595569 (closed) for context.