Log camera device events

[joseph-reynolds]

This issue to to create a usable set of logs for each camera device.

Each camera device should keep its own log accessible from journalctl ("sudo journalctl _SYSTEMD_UNIT=snapaway-cam.service"). This is already partly working. We need to log more events, and have a way to format the events for viewing. Events are:

• camera on (started monitoring for motion)
• camera taking picture (perhaps also "why", for example, if we ever have an external motion sensor)
• camera off (turned off manually; we may not get this if we unexpectedly lose power)
• perhaps an "i am alive" log every hour
• updated settings (e.g., motion sensitivity, rotation)
• local web app server online/offline (probably don't need to log when pictures are accessed)

Log rotation is handled by the operating system.