Add alive signal to monitor that the service is up and running

[AndBobsYourUncle]

Recently, I've noticed that the service had gone down due to some issue, whether it was a network issue, or it was a cat bumping into the USB Neural Compute Stick and causing it to crash.

This adds a some new configuration for both a LWT (last will and testament), as well as topic for the alive status of the app.

What this will allow is the automation of a system alert, broadcast to my media devices in Home Assistant, that lets me know when it has not been running for some length of time.

Also, it fixes a bug where the initial "OFF" state was not getting broadcast to all camera topics.