Deny unknown fields in users

[niclaflamme]

Context

• Discovered that it was almost impossible to fail a /users.toml validation because these extra values just get ignored.
• This means that pgdogdev/docs cannot be validated.
• This is because every docs' toml snippet has to either pass users.toml validation or pgdog.toml validation.
• Without #[serde(deny_unknown_fields)], it's very hard to fail a users verification.
• The pgdog.toml snippet is not labeled, so the broken pgdog.toml was being accepted as a users.toml.

[invalid_field]
always_passes = true

Changes

• add #[serde(deny_unknown_fields)] to config::Users