Add Carto API key for maps #167

Our basemap tiles are displaying watermarks indicating that we need to add a Carto API key to our code:

See https://carto.com/basemaps/apikey/ for an explanation of what this means, and how to request an API key. Once we receive an API key, we'll need to add it to our code.

Some notes and open questions:

• We should make sure to restrict the authorized referers for this key to ensure it is only valid on the HomeVal prod and staging sites (as well as localhost). Carto calls this option "Restrict to specific websites (Referer)".
• I'm guessing that we'll need to expose our key in the code, since HomeVal is deployed as a static app. This isn't great, but it should be mitigated by restricting the valid referers for the key (see the bullet above). We should double-check my understanding here, however, since it's possible there's a way for us to obscure the key.