Add Twitter and Facebook cards (`<meta>` tags)

[cvan]

à la mozvr.com's:

    <meta property="og:title" content="Mozilla VR">
    <meta property="og:site_name" content="Mozilla VR">
    <meta property="og:type" content="website">
    <meta property="og:url" content="https://mozvr.com/">
    <meta property="og:description" content="Bringing Virtual Reality tools, WebVR standards, and experiences to the open Web.">
    <meta property="og:image" content="https://mozvr.com/assets/img/card.png">

    <meta name="twitter:card" content="summary">
    <meta name="twitter:site" content="@mozillavr">
    <meta name="twitter:creator" content="@mozillavr">
    <meta name="twitter:title" content="Mozilla VR">
    <meta name="twitter:description" content="Bringing Virtual Reality tools, WebVR standards, and experiences to the open Web.">
    <meta name="twitter:image" content="https://mozvr.com/assets/img/card.png">

[cvan]

Additionally, generate them for individual browser and headset pages.