MQTT is now broken

it now appears wifi_util was broken.

* Night mode is still available when pressing the flash button
* "led 0" or "led 1" should now be used as commands instead of
"night_mode"

e.g. for local MQTT on port 1883
encryption will be on by default

A short red wheel is displayed after the LED countdown has been
completed. It prevents a new countdown from starting, or night mode to
be activated.

Since a red wheel is displayed at the start of the calibration process
anyway (until measurements are stable enough), it should hopefully not
be too surprising.

* The 'int' return signature and the comment
'Returns the number of remaining leds' sounds like the
function would return its current countdown state.
However, it only returns the number of remaining
leds after the button was released. Thus change
the type to 'bool' and return 'true' after a
successful countdown

* Do not use 1 as return value for night mode,
because it is ambigous. Return 'false' instead.

co2_sensor: Make co2 alert threshold configurable

See merge request !11

Previously, it was hardcoded to 2000 ppm, which
remains the default value.

Clarify documentation of `reset` command

See merge request !9

The old wording 'sensor' for the whole controller can be
misleading.

If calibration is finally reliable, let's update the version!