Add supports of adding/removing sensors.

[yiskang]

Hi Team,

This PR includes the following changes:

• A JsonDbSyntheticGateway that uses lowdb to store DeviceModels and Devices data in a local JSON DB and provides functions of adding/removing Devices from the DB w/o restarting the app.
• A JsonDbRestApiDataAdapter that supports adding/removing Devices.
• A viewer extension SensorManagerExtension to interact with the above w/o using React.js.
• A new page SensorManage demonstrates the functionality of the above. It's similar to what Playground does.

Cheers,