Create .gif for Facebook and sharing preview

[BenMoses]

Facebook supports gif format for sharing. We should try to implement this for first launch, a single gif, of the letter / envelope.

This gif also needs to be created for Emails.

<meta property="og:url"         content="url to GIF on web">
<meta property="og:title"       content="Title of GIF page">
<meta property="og:description" content="Some description">
<meta property="og:type"        content="video.other">
<meta property="og:image"       content="Same as og:url above">
<meta property="og:image:width"  content="800">
<meta property="og:image:height" content="400">```