Font toggle not working in `diffbrowsers_user_strings.html`

### Issue description:

The user strings are added as 'divs' with class names created using the `font_class.class_name` string formatting on [line 12](https://github.com/googlefonts/diffenator2/blob/69a873d79811e957aa5824e04d4859717f206c47/src/diffenator2/templates/diffbrowsers_user_strings.html#L12) (for e.g. `old-Bold`, `old-Regular`, etc.).

These classnames are not targetted by the `switchFonts` function and as a result don't update when toggling.

### Possible solutions:

1. Modify line 12 to use the `box-text` class as the main class with the class name formatter string being the sub-class.
I.e.: `