Add --clean option to ./bin/deploy.py

[metasoarous]

Ideally, we wouldn't leave old data lying around from old deployments, for storage costs if nothing else. (Also for removal of accidentally pushed (or retracted) data).

Idea here would be to use boto3 to survey the bucket for content that didn't match the content which was just pushed. May pay to be smart here about cleaning out data vs app assets, depending on the update scope (full vs app vs data).