Summary: With the new design from vpileggi, this diff separated a new app UI to the current one. This diffs show the toggle switch to enable Sandy UI for devs who are in `flipper_sandy` GK. When toggled, it will bring Sandy UI up. Reviewed By: mweststrate Differential Revision: D23599398 fbshipit-source-id: d85c707e0fe7726a418b3551cedb36e455fb7d14