Battery charge indicator

[fweiss]

There is a BT battery characteristic from a earlier version, maybe Sparkfun ESP32 Thing. But on the current "LOIN" board, the ADC pin is not connected to the battery.

• connect the ADC to battery with voltage divider
• create some other indicator, may a led pattern, to show low battery

This is related to #12. One of the problems is that when battery gets low the BT fails and there can also be a WDT reset. Some ideas:

• check for battery low and turn off BT or don't initialize it
• show a battery icon in place of the animation

The main idea is to not malfunction and notify the user that the battery needs to be recharged.

[fweiss]

Maybe also run a test to estimate how long badge runs with a fully charged battery and also how long it takes to charge.

[fweiss]

Also fix this warning:
warning: 'ADC_ATTEN_DB_11' is deprecated [-Wdeprecated-declarations]

[fweiss]

Also see #13. We've seen some WDTs and/or brownouts when running on battery. This is may be related to bluetooth as well.