Move away from Google Maps for directions service

[pezholio]

If there's a lot of routes on a page, we get this error:

This is because we calculate the routes via the Google Maps API every time we display a route on the frontend, and we're hitting the rate limit. I've done a temporary fix by logging the error to the console, rather than alerting, but we need to do two things here:

1. Move away from the Google Maps Directions API, and use Mapbox, which we use elsewhere anyway
2. Cache the route so we're not hitting the API every time.