WIP Fix twitter cards

[rachelwhitton]

Fixes #2419

Summary

Copies the source/images/assets directory to the static/images/assets directory (related Gatsby doc)

Fixes meta image tags across all templates, currently they're 404s, for example:

<meta name="og:image" content="https://docs.pantheon.io/images/assets/default-thumb-doc.png" data-react-helmet="true">

• Todo: audit all templates to make sure we're using the appropriate image, for example it likely doesn't make sense for the certification template to use a Terminus image

<meta itemprop="image" content="https://docs.pantheon.io/images/assets/terminus-thumbLarge.png" data-react-helmet="true">

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[pantheon-decoupled]

⚡ Deployed with Pantheon Decoupled

This build was successfully deployed with Pantheon. You can track the build logs here.

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