Validate outlook.office365 FQDN in webhook URLs

Based on research/testing, it appears that both of these two
FQDNs are interchangeable:

- outlook.office.com
- outlook.office365.com

This commit updates the isValidWebhookURL validation
function to accept either one as a valid webhook prefix.

refs dasrick#3

send.go

@@ -82,8 +82,10 @@ func isValidWebhookURL(webhookURL string) (bool, error) {
 		return false, err
 	}
 	// only pass MS teams webhook URLs
-	hasPrefix := strings.HasPrefix(webhookURL, "https://outlook.office.com/webhook/")
-	if !hasPrefix {
+	switch {
+	case strings.HasPrefix(webhookURL, "https://outlook.office.com/webhook/"):
+	case strings.HasPrefix(webhookURL, "https://outlook.office365.com/webhook/"):
+	default:
 		err = errors.New("unvalid ms teams webhook url")
 		return false, err
 	}