New API contract [ /api/things ]

[dillonfagan]

GET /api/things?p=0

{
    "DIY": [
        {
            "id": "...",
            "title": "Hammer",
            "description": "...",
            "categories": ["DIY"],
            "images": ["https://..."]
        }
    ],
    "Outdoors": [],
    "Sports": []
}

To simplify our client-side logic and to prep for pagination, we should introduce a new contract for retrieving all things in the library's collection, organized by category.