Align social meta with live except image. (#999)

Some faff because og:image must be absolute.

Author: microbit-matt-hillsdon

public/index.html

@@ -6,11 +6,13 @@
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <meta name="theme-color" content="#6c4bc1" />
     <title>Python Editor for micro:bit</title>
-    <meta property="og:title" content="Python editor for micro:bit" />
+    <meta property="og:title" content="Python Editor for micro:bit" />
     <meta
       property="og:description"
       content="Built by the Micro:bit Educational Foundation and the global Python Community."
     />
+    <meta property="og:image" content="https://python.microbit.org%PUBLIC_URL%/social.png"/>
+    <meta name="twitter:card" content="summary_large_image">
     <meta
       name="twitter:description"
       content="A Python Editor for the BBC micro:bit, built by the Micro:bit Educational Foundation and the global Python Community."