Wrong thumbnail for social media sharing on all landing pages

When you share the landing pages on social media, they are all using the "Your entire infrastructure. Defined as code. In about a day." purple image from the gruntwork.io main website. This is because all the pages are reusing the header from the main website, which hard-codes the `twitter:image` and `og:image` meta tags to this image. We should have different images for each page.