Automatically invalidate cloudfront cache on deploy

[metasoarous]

Ideally, deployment would be instantaneous, but presently you have to wait for cloudfront to update its cache before any changed content on www.olmstedviz.org would become available. This can be amended by invalidating the cache, which can be done manually, but is annoying. Ideally, we'd just use boto3 (which we use presently in the deploy script) to do this every time we deploy.

[eharkins]

https://docs.aws.amazon.com/cloudfront/latest/APIReference/API_CreateInvalidation.html