Replace all <span>...</span>s with annotations

This involves post-processing the docs using JavaScript. It is a known bug in scaladocs that the summary pages cannot display both annotations and comments at the same time. A workaround for this is to introduce an identifier tag (e.g. `:: DeveloperApi ::`) that we grep for to add the badges later manually. As soon as the scaladocs bug is fixed, these semi-ugly tags will go away.