config: add www_folder to default config

[missytake]

Maybe it should be discussed actually:

• on cmdeploy init, do we want to add all possible config values, or only the necessary ones?
• if we add config values with a default value, should they be commented out?
• where do we document config values? Right now most are documented in chatmail.ini.f, and some in README.md. The former is not obvious for people who upgrade their relay to a version with new config values, the latter might be a bit harder to find for new users.