Add folder view component

[Carifio24]

This PR adds an implementation of the folder view component. This is an expanded version of the component that we've used in previous stories (the Rubin story in particular). The main items of note are:

• We've never really used the folder navigation piece of this before, and so while testing it out I unsurprisingly found a few issues. This component has those resolved. The Storybook story for the component in this PR uses the same folder as the WWT webclient so one can easily see this functionality at work.
• This component has built-in default content for the folder items, but in case we want to tweak what the item display looks like for a given story, the item content is now in an item slot. The downside to this is that the slot content will need to re-implement the selection handling itself. I would've liked to push the event handlers up to the slot container, but this could potentially interfere with the functionality of whatever content one puts into the item slot.
• As a convenience, the root folder can now be specified as a URL instead of passing in a Folder object, if one wants. If managing the folder instance isn't necessary in the parent component, this can help reduce overhead on that side.

There are still some CSS issues to work out here, but I think the internals of this are in good shape.