Added additional error info when testing, custom events, and "Do nothing" action

[derekantrican]

This PR adds 3 small changes:

• Additional info (via the browser's console) when clicking "Test sensor"
• Firing custom events when the sensor is triggered (useful for plugins like https://github.com/tjjfvi/OctoPrint-IFTTT that allow you to subscribe to custom events)
• A 'Do Nothing' action option to perform when the sensor is triggered (useful for users who only want the navbar status & popup notification or for users who will use the custom events mentioned above to create their own action/notification)

[derekantrican]

@valemaio2 I think I'm going to have to move my PR to your repo