Add device register model, controller, and routes to backend

[iamnwi]

Model:

• Device name is unique for devices under a user
• Use userId + (deviceName or deviceId) to identify a device

Routes:

• GET /api/v1/users/:userId/devices
• POST /api/v1/users/:userId/devices
• DELETE /api/v1/users/:userId/devices/:deviceId

[iamnwi]

IBM Cloud toolchain: Delivery Pipeline deployed ExpenseTrackerApp to mycluster, including commit d9d404477fe2293ce09ab35edf0b3fc216d1d948