Add SMTP backend for sending emails

### Problem being solved

SendGrid and SES are the only backends, so self-hosters without either cannot send mail.

### Proposed solution

Add nodemailer and support sending emails through SMTP as well 

### Alternatives considered

NA

### Anything else

_No response_

### Screenshots

_No response_

### Checklist

- [x] I’ve reviewed and agree to follow the code of conduct