replace glue with htmltools

[kbodwin]

Suggested by @gadenbuie :

On that last point, it'd be worth thinking about using htmltools instead of glue to create the HTML tags. One problem that could arise with the current approach is that it's possible to escape the style string and open a new attribute or tag. I mean, I doubt anyone is really running into this, but it could create odd bugs down the line. (It's how I was doing this class business before this PR.)