Add support for settings in non-volatile memory, configurable via web interface

[maehw]

Let me know if you need a clearer summary as there are more changes (older ones) than the title suggests.

You could then do some cherry-picking.

[kmetz]

Thank you!
I'm planning to merge back most of it, with continued support for platformio, BME280, and some small changes (like allow warning buzzer to buzz infinitely), will need a bit of time. So far everything is clear.