Bulk addition and changes: More LEDs; add fan and CO2 sensor; refactoring

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The code can now use more hardware:

• Arrays of LEDs (4 configured by default),
• A 5V fan that continuously runs. No control at this point,
• An MG-811 CO2 sensor.

The data is also added and published to the backend, together with the data so far (snapshot, humidity, temperature, LED states).