Clean white theme in black and Bristol blue, move to webhook.me.uk

The landing page and the email shell drop the dark gradient look for a
white background, black text and a single Bristol blue accent. Copy is
unchanged. Email footers and docs now point at webhook.me.uk; the
dixon.cx hostnames keep working.

WEBEX_INTERACT_GUIDE.md

@@ -15,7 +15,7 @@ https://your-domain.com/{your-email}/wxinteract
 ### Example
 
 ```
-https://webhooks.dixon.cx/sms-notifications%40mycompany.com/wxinteract
+https://webhook.me.uk/sms-notifications%40mycompany.com/wxinteract
 ```
 
 ## Supported Events