Use icon GIF for packages panel: Add horizontal scrollbar if required; Update text for nav links

[shu8]

This PR mainly affects the packages panel on the /edit.html page

• Adds a horizontal scrollbar if there is more content to be seen
• Uses the package's icon GIF for the XRPK rather than the question mark icon placeholder:

For reference, this is what it looks like right now:

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I've also updated the navigation bar items to use the correct text, e.g. Switch to edit mode if you're on the homepage, rather than You are currently editing this world, which should only be there when you're on the /edit.html page

[avaer]

Also I'm thinking that inventory list will probably make more sense with a vertical scroll and a search bar, but that's a future PR.

[shu8]

Thanks for the heads up! and sure, search bar + vertical scrollbar might make more sense :)