Constant location pings drains mobile device battery too quickly; adjust resource utilization based on battery level about buoy

[fabacab]

This is dramatically helped by the fact we've implemented server-sent event streams in betterangels/better-angels#170 but an explicit battery level check could still be a useful feature.

[fabacab]

Newer browsers have sensibly begun restricting the use of Battery Status API to privileged "chrome" code only (i.e., code that runs in an extension rather than JavaScript running on a page). This began with Firefox 52; see its release notes. So it looks like this isn't something we're going to be able to do with just a web app. As such, closing this issue.