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.

@marcotuts at a glance it's possible, but would require some improvements that can stretch out this Content Libraries MVP beyond deadlines. There are some shortcuts taken in this PR to avoid heavy refactoring the way child XBlocks are rendered and passed to studio, which likely will work only with single (per unit) top level XBlock. This line probably one of them.

So, while adding paging capabilities to XBlocks sounds like a valuable addition, I suggest it's left outside of scope of Content Libraries MVP, and implemented later. I've sent a [message](https://groups.google.com/forum/#!topic/edx-code/XudnbUC3mdQ) to edx-code ML describing current status and what needs to be done in order to reuse this.

On the other hand, if I slightly misunderstood you and your suggestion was only to move paging header inside XBlock template, but not prepare for recursive building, it's more or less doable in terms of this PR. It just adds little value, since LibraryRoot will always be the only XBlock on page, while might take significant effort to implement. @Kelketek what do you think?