Centrally define and validate environment variables

[harpojaeger]

Ref MoveOnOrg/Spoke#1058. @jlegrone, would this want to wait until further work is completed on Docker/Tilt stuff?

[jlegrone]

@harpojaeger no I think we're good to go now that #5 is merged.

Validation and configuration management seemed like distinct issues to me at first, but after looking at envalid it seems like the schema it provides would probably be enough to do code generation for .env files, Heroku's app.json, lambda-env.json, etc. So I think a first priority should be to look at validation options including envalid with code generation in mind.