Add SEO metadata

[KyleMcMaster]

• Add metadata from the site to seo.js to allow for the population of featured images, author images, and other content when sharing URLs on social media like twitter for example.

We'll probably want to update the helmet component for similar attributes to this:

    <Helmet
      htmlAttributes={{
        lang,
      }}
      title={title}
      titleTemplate={defaultTitle ? `%s | ${defaultTitle}` : null}
      meta={[
        {
          name: `description`,
          content: metaDescription,
        },
        {
          property: `og:title`,
          content: title,
        },
        {
          property: `og:description`,
          content: metaDescription,
        },
        {
          property: `og:type`,
          content: `website`,
        },
        {
          property: `og:image`,
          content: featuredImageUrl,
        },
        {
          name: `twitter:card`,
          content: `summary`,
        },
        {
          name: `twitter:creator`,
          content: site.siteMetadata?.social?.twitter || ``,
        },
        {
          name: `twitter:title`,
          content: title,
        },
        {
          name: `twitter:description`,
          content: metaDescription,
        },
        {
          name: `twitter:image`,
          content: featuredImageUrl,
        },
      ].concat(meta)}
    />