Resolves https://github.com/coder/coder/issues/15513
Disables notifications when both `$CODER_NOTIFICATIONS_WEBHOOK_ENDPOINT` and `$CODER_EMAIL_SMARTHOST` are unset.
Breaking change: `$CODER_EMAIL_SMARTHOST` is no longer set by default as `localhost:587`, meaning any deployments that make use of this default value will need to add it back.
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Co-authored-by: Danny Kopping <danny@coder.com>
Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>
