#a11y: Add ALT text to images

[hejjoe]

It should be possible to add an alt text to tweets, which are created via the twitter api. Ideally the alt text contains all the words of the text snippet in the uploaded image. https://blog.twitter.com/developer/en_us/a/2016/alt-text-support-for-twitter-cards-and-the-rest-api https://developer.twitter.com/en/docs/twitter-api/v1/media/upload-media/api-reference/post-media-metadata-create