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.

One question here around where this is being injected into our templates. Considering @andy-armstrong's work on recursive building of xblock pages, would it be fair to think of the xblock-container-pagination control as something that optionally can be added to any xblock? Currently xblock-messages can be shown at page levels, component levels, and if the outline is also set up to be nested, potentially also at those levels.  Independent of whether this should be a general pattern to optionally add to the header/message area of xblocks, it would still be great to render this area slightly differently (as described below.) 

I asked the above question in part because the existing page pattern for pagination (from Files & Uploads) adds in pagination as a message at the page label, which associates the label with the header as opposed to the content elements within the xblock, which makes it a bit clearer what the pagination is impacting. screenshot: https://www.dropbox.com/s/qdl8dx345ve1tc2/Pagination.png?dl=0

Depending on the level of effort, it would be great to consider having this pagination be something that is shown in gray within section class="wrapper-xblock level-page studio-xblock-wrapper" but before div class="wrapper-xblock-message xblock-validation-messages")