better configure social sharing

[aboydnw]

1. define the default url for sharing in social apps
2. define the preview image for posts on social apps

This code block in index.html

    <meta name="author" content="Development Seed">
    <meta name="description" content="An interactive visualization of contributors and their connections to repositories">
    <meta name="theme-color" content="#cf3f02">
    <meta property="og:title" content="The Development Seed Contributor Network">
    <meta property="og:url" content="https://developmentseed.org/contributor-network/">
    <meta property="og:type" content="article">
    <meta property="og:locale" content="en_US">
    <meta property="og:image" content="https://developmentseed.org/contributor-network/site-image.jpg">
    <meta property="og:image:type" content="image/jpeg">
    <meta property="og:image:width" content="1200">
    <meta property="og:image:height" content="800">