Add key for Google Maps Embed API

[willgearty]

The Google Maps Embed API is entirely free, so we shouldn't require chapters to supply their own key. We'll need to include it in local_settings.py (like with the reCAPTCHA key) so we don't reveal it to the world on GitHub.

However, the other tricky thing is that we would also ideally have this hidden somehow within the template, otherwise it could be scraped from the HTML of the onsite pages (although only signed-in users would be able to see this page, so maybe it's not a huge concern after all).