Add configuration to display narrative distances in metric units

[binh-dam-ibigroup]

This issues originates from this suggestion to add a config parameter to display narrative distances in metric units. Currently, distances are hard-coded to be shown as imperial.

This issue depends on otpui/#116.

[nukeador]

Just checking on this. Since I see there is a path on one of the forks, is this something you are interested in integrating here?

[nukeador]

I think this keeps being blocked by this pending PR for otp-ui to fix @opentripplanner/itinerary-body

opentripplanner/otp-ui#736

Also at otp-react-redux level, lib/components/viewers/nearby/distance-display.tsx has hard-coded imperial units here

localizedDistanceString: humanizeDistanceString(distance, false, intl)