Fix Safari favicon once they X it

[insin]

They've either forgotten or didn't know to change this one for Safari:

<link rel="mask-icon" sizes="any" href="https://abs.twimg.com/responsive-web/client-web/icon-svg.168b89da.svg" color="#1D9BF0">

Current file:

This is why you don't get a pip on the favicon in Safari when you have notifications, as the mask-icon can only be an SVG with a single colour - we might be able to improve that by swapping in a different version or changing the colour when there are notifications, but I don't know if Safari applies those changes immediately.