Add OG images for articles

OG images for articles are built according to the design.

To consider:

Eleventy in dev mode terminates if no square image for the post is found. I can set a fallback, but it's important to get notified of that -- otherwise you might only realize that after you shared the article.
if no lede is set like in developers-appetite.md, there is no fallback in place (layout does not change). I remember you said there will always be a lede value, so that's no problem I guess.

New filters:

base64Format The images for sharp have to be encoded as base64. For now, this filter only handles jpg, jpeg and png formats.

getInputDir The Eleventy page object gives me the whole inputPath with filename. I remove the file name and just keep the path do the directory.

appendSuffixToFilename Appends the given suffix to the file name.

Name	Link
Latest commit	beb517b4199a81630dc0bf497df6fed6d32f3b65
Latest deploy log	https://app.netlify.com/sites/unlockopen-com-v4/deploys/665efde5d78a620008195816
Deploy Preview	https://deploy-preview-36--unlockopen-com-v4.netlify.app
Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.