[PR] Added OGP images.

Summary:
I added two [Open Graph Images](http://ogp.me#array) of the sizes recommended by the [Facebook Developer documentation](https://developers.facebook.com/docs/sharing/best-practices#images) using the SVG icon from the website's header and with the background colour of the header.
I added these to the default layout so they should be crawled on any page.
Closes facebook#3807

Reviewed By: mroch

Differential Revision: D4976482

Pulled By: gabelevi

fbshipit-source-id: d8edf3a417c9d7840c7e5a1c365abad8eeaa022a

website/_layouts/default.html

@@ -9,6 +9,14 @@
     <title>{{meta_title}}</title>
     <meta name="description" content="{{meta_description}}">
     <meta property="og:url" content="https://flow.org{{ page.url }}">
+    <meta property="og:title" content="{{meta_title}}">
+    <meta property="og:site_name" content="Flow">
+    <meta property="og:description" content="{{meta_description}}">
+    <meta property="og:image" content="http://flow.org/static/ogp-1200x630-1.png">
+    <meta property="og:image:secure_url" content="https://flow.org/static/ogp-1200x630-1.png">
+    <meta property="og:image:type" content="image/png">
+    <meta property="og:image:width" content="1200">
+    <meta property="og:image:height" content="630">
     <link rel="canonical" href="{{meta_canonical}}">
     <link rel="alternate" type="application/rss+xml" title="{{site.title}}" href="{{site.url}}/blog/feed.xml" />
     <link rel="shortcut icon" href="/static/favicon.png?t={{site.time | date_to_xmlschema}}">