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.

@andy-armstrong Yes, I'll fix that.

@cahrens This isn't the view, but the page. The page decides which type of container to instantiate, be it paged or unpaged.

...Interestingly, though, if this instance gets changed to 'paged_container', then at the moment we'll have no uses of the library container, and it will just be an unused file. I'm still fuzzy on the architecture here. There may be a better way to handle this, but I'm not aware of it.