Remove labelString from ListboxItem

[charliepark]

The original intent of the labelString was that it would enforce a single line for the select box. Having a different component for the pseudo-select (button) and the pseudo-option (div) complicates things, though. If we must enforce single-lined-ness, we can do so with a non-wrapping flexbox, but we should be able to evaluate from the various callsites whether that's needed or not.

This will require going through a few locations in the app where we use it and making sure there aren't unintended consequences, but we can simplify how we're handling our listboxes.

[charliepark]

There's another usecase that perhaps justifies having two different props here.

Consider this scenario:

Essentially, we have a "summary" template that we use when the item is selected, and then a more complete template that we show in the dropdown box.

I think we might be fine to keep the concept of the labelString, but change the prop name to something a bit more descriptive of its purpose, and less tied to its type. So something like labelString → selectedLabel and (maybe) label → dropdownLabel. That way, we have a more clear sense of the context where the label will show up.