[MM-16146] When converting a bot to a user, set the role to system_user (#11361)

* when converting a bot to a user, set the role to system_user

* correct govet shadow error

* Change from inline error checking and fix shadow error

* Add SYSTEM_USER_ROLE_ID to the bot user model instead of updating the
role after the bot user is created

* modify UserFromBot test to include Roles field

model/bot.go

@@ -169,6 +169,7 @@ func UserFromBot(b *Bot) *User {
 		Username:  b.Username,
 		Email:     fmt.Sprintf("%s@localhost", strings.ToLower(b.Username)),
 		FirstName: b.DisplayName,
+		Roles:     SYSTEM_USER_ROLE_ID,
 	}
 }