public.json: Add /categories and /category/{id}

Based on the schema Jordan and Casey have outlined [1]:

  {
    id:
    name:
    canonical_uri:
    slug:
    image:
    item_count:
    parents: []
  }

Changes from their schema:

* Dropped 'canonical_uri', because I'm not convinced HATEOAS is
  worth it (see azurestandard/beehive@deb2e28f,
  apps/api/views/drops.py: Adjust to match the current API spec,
  2014-11-20).
* Dropped 'slug', because I'm not sure why we'd need this.
* Changed 'image' to an 'images' array to match products.
* Dropped 'item_count', because we should be able to count
  products in this category by searching /products?category=<id>.
* Converted 'parents' to 'parent', because we want a category
  tree, not a more generic category graph.

[1]: https://github.com/azurestandard/website/blob/71f01fa4cc41acbfd3abb921b22875ea15c069e3/docs/api.md#product-1
public.json: Change string -> int64 when filtering by ID