Added file-based logging sub-system. Also moved various WEB strings to PSTR

This version adds functioning logging sub-system. Two types of logs are
enabled - system log (regular events, such as device start), and
watering events - to facilitate watering activity logging.

Watering events are exposed in a way to support regular sprinklers_pi
Logging UI. Currently only Table logging UI is working, Graph will be
coming in later.