When setting text in the DOM, use textContent instead of innerHTML

See the MDN docs on some of the downsides of setting things with innerHTML here.

This shouldn't be a huge problem since Empress visualizations are (as of writing) inherently client-side applications, but there's the potential for users to break things if their data includes bizarre names -- for example, a metadata column is named <b>i'm a problematic metadata column</b>, or something silly like that. (Also, more realistically, backslashes or ampersands might also cause problems with innerHTML.)