Create API endpoint(s) for nested, paginated topic tree data retrieval

[rtibbles]

The new channel display for 0.15 requires showing multiple levels of the ContentNode hierarchy at once, and also for properly paginating the contents of these topics.

Using our current APIs this would result in requests to:

• Get the parent topic
• Get the child topics (up to the pagination limit - N)
• N requests to get the paginated children of each of the child topics

This issue proposes instead to create a single endpoint that returns all of this information in a single request. As this will be the primary means by which the topic tree is navigable, this will still have the virtue of being easily cacheable and reusable across multiple clients.