Change "gmail" to "smtp" so we have more generic env names

This also uses env.sample instead .env-sample because .env-sample results in a hidden file on Linux, making it hard to see (in fact, I didn't even realize it was there until a few minutes ago). The samples should be easy to find and not hidden.

Don't merge this until Briang's changes to the usage of the env is merged too so we stay in sync.