When expanding objects in an API response, the caller loses out on some

useful information, such as the original URL to the expanded resource

and the mimetypes. This causes the caller some extra work if they need

this information, either making assumptions about URLs/mimetypes or

performing a second request to get these details.
This change updates the payment to provide some useful information on

the expanded information. Payloads that expand resources now have an

_expanded field that has each expanded field name as a key and

information as the value.
If expanding a model (such as from a ForeignKey), this will contain

an item_mimetype key mapping to the mimetype of the expanded resource.
If expanding a QuerySet, this will also just contain item_mimetype

(as there's not necessarily a corresponding list resource that the user

could otherwise get access to).
If expanding a child resource, this will contain item_mimetype,

list_mimetype, and list_url, all of which can be useful for mapping

the results back to native objects on the client.

Unit tests pass in both Djblets and Review Board.
Manually tested this with some resources, making sure I always got the

results I expected.