Improve battery management of T-Stick

[aburt2]

• Better feedback for when a T-Stick is on/off
• better battery state feedback (current signal is unreliable and requires manual tuning when battery is replaced)
• replaceable batteries
• Update build guide to state the batteries need to have an on cell protection circuit as the tinyPico's BMS doesn't have one

[aburt2]

Added MAX17055 fuel gauge, current results indicate good accuracy for battery percentage estimations