Include total app count in homepage serializations of collections (needed for bug 1052858).

r? @ngokevin @robhudson

Docs: we don't document this particular serializer (which we should), so I'm not making changes. I will file a bug to make sure that we do ultimately document them.

There are other homepage serializers that truncate apps. There's an argument to be made that we should also include this field in those serializers for completeness' sake, but these serializers are so specific that I don't see any point in doing so. Happy to hear arguments against that notion, though.