If org with provided slug doesn't exist or no public collections
exist for that org, return same 404 response with a detail of
"public_collections_not_found" to prevent people from using public
endpoint to determine whether an org exists.

Endpoint is /api/public-collections/<org-slug> to avoid collisions
with existing org and collection endpoints.

This will be more easily extensible if we want to add additional
fields (like an organization description) that would be visible in
a public profile.