Move the collection parsing into `Metabook.fromCollection`.

In the process, use the `Parsoid` class for doing parsoid requests
and resolve hrefs in a slightly more robust manner.

Also tweaked the special case for wikibooks.

Change-Id: I83366997ac5ef577ca4d28d5f033c4a7696c4b15
Merge "Move the collection parsing into `Metabook.fromCollection`."