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.

Aren't these simply tests of PagingHeader (which has its own unit tests-- see paging_spec.js)? I don't see that this is testing anything specifically about this page. There is no need to duplicate testing that PagingHeader and PagingFooter work unless there is something specific about the paging container behavior that needs to be tested.

Looking further at paging_spec.js, I see that the test there is intermingled with assets. It really shouldn't have been. There is no need for you to refactor that test, but there is also no need to make a copy of it that deals with the container page.