Feature: ignore availability topic option for sensors

[amadeo-alex]

This PR:

• adds an option during sensor configuration to ignore the availability topic
  • this causes the sensor to be viewed by HA as always available and presenting last known value, even if HASS.Agent is offline
  • an information message is displayed for the user when ignore availability option is selected

Should remediate:

• Bug: All entities unavailable once HASS.agent disconnects/machine is shutdown HASS.Agent#301
• Bug: with _monitorpowerstate when pc turned off showing Unavailable HASS.Agent#303

---

Message presented to user when ignore availability option is selected:

HA after the dummy sensor has been configured to ignore the availability: