Allow default SiteSettings lan/lon to read from environment variables

[JosiahDub]

The SiteSettings lat/lon can easily be changed in Admin once the site has been set up, however if a default lat/lon for a deployment is configured, the lat/lon can be set to that at the beginning, saving a set.

The model and migration file default would read an env var like DEFAULT_LAT. This variable can be reused in multiple places.

[jacobwhall]

Yeah, I haven't yet decided what should be defined in the SiteSettings model vs. defined by the deployment parameters / environment variables. In general I think site content / behavior should be in the former, and deployment details such as s3 bucket keys in the latter.