Add support for Content-Security-Policy in images

This commit adds support for the "Content-Security-Policy" header in the
barrel images.

internal/wisski/ingredient/barrel/system/system.go

@@ -32,6 +32,8 @@ func (smanager *SystemManager) Apply(ctx context.Context, progress io.Writer, sy
 		return err
 	}
 
+	// TODO: Apply Content-Security-Policy!
+
 	// and rebuild
 	return smanager.Dependencies.Barrel.Build(ctx, progress, start)
 }