*   Add API commands for daily and global leaderboards.
*   In the process, the Servant API is reshuffled a bit: `Articles` has been
    generalized to `HTMLTags "article"`, to also support `HTMLTags "div"`.
    `FromArticle` is now `FromTags "article"`.
*   Move some of the data types to be in their own module, *Advent.Types*.