feat: implemented smtp service;

feat: implemented registration emails;
fix: config variables for password length used the same env variable;
refactor: all available config variables added to docker-compose.yml

backend/internal/errors/smtp.go

@@ -6,5 +6,5 @@ import (
 
 var (
 	ErrSmtpDisabled = errors.New("Smtp is not enabled in the config")
-	ErrSmtpSendFailure = errors.New("Failed to send email")
+	ErrSmtpMissingConfiguration = errors.New("Some necessary SMTP configuration is missing")
 )