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.

I'm not sure. This is @e-kolpakov 's original implementation. I can take a crack at swapping it out with the backbone implementation. It may be that he thought that it wasn't available to him and he didn't want to add a JS library for this one thing. He's probably asleep right now, but perhaps, after I've hammered on it for three hours and given up, he'll comment to explain why I should have left it that way. :)

I'm going to give it a shot, though.