Implement Pagination for Content libraries (SOL-4)

* **Background**: This PR provides pagination for content Libraries in the container view, when displaying the blocks contained in the library.
* **Jira tickets**: Implements SOL-4
* **Discussions**: Architecture discussed extensively on the wiki and in meetings, then the revised proposal was presented to the Arch Council on Oct. 21 and given thumbs up.
* **Dependencies**: https://github.com/edx/edx-platform/pull/6046 (merged, in feature branch https://github.com/edx/edx-platform/commits/content-libraries)
* **Sandbox URL**: http://sandbox.opencraft.com:18010/library/library-v1:TST-X-01+now
* **Internal code reviews PRs**: https://github.com/open-craft/edx-platform/pull/9 https://github.com/open-craft/edx-platform/pull/18
* **Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.

@cahrens and I were just discussing this. It is a shame that you've had to duplicate all the logic from PagingView because you can't use multiple inheritance. I'd prefer to see all the logic moved out into a paging_utils class, and then have both PagingView and your new view use the shared methods. The other option would be for your view to subclass PagingView, and to use helper methods to render the xblocks. I'm not sure which would be easier/cleaner, but having duplicated code doesn't seem like the right answer.