Add get default CO2 recalibration value API endpoint and consume in calibration UI

[sjefferson99]

Add a method in the SCD30 module to retrun the default calibration value set in config.py
Adjust the manage SCD30 UI page to default to the config value instead of 400, but keep a note that fresh air is around 427ppm and can be checked at https://www.co2.earth/