LibraryContent - Display content from a library in a course (SOL-5, SOL-6, SOL-7, SOL-8, SOL-117)

* **Background:** This PR contains the LibraryContent XBlock, which allows to display library content in a course.
* **Jira tickets:** Implements SOL-5, SOL-6, SOL-7, SOL-8., SOL-117.
* **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)
* **Partner information:** 3rd party-hosted open edX instance, for an edX solutions client.
* **Merge deadline:** Dec 12
* **Sandbox URL**: LMS at http://content-libraries.sandbox.opencraft.com/ and Studio at http://content-libraries.sandbox.opencraft.com:18010/
* **Internal code reviews PRs**: https://github.com/open-craft/edx-platform/pull/5 https://github.com/open-craft/edx-platform/pull/3 https://github.com/open-craft/edx-platform/pull/2 

**Instructions to test:**

1. Create a Library with some content, and a course
2. In the course, go to Settings > Advanced Settings and add `"library_content"` to advanced_modules.
3. Go to Content > Outline. Create a new section and subsection, then edit it. Click "Advanced" and add a new "library_content" block.
4. Once the block is added, click "Edit" to edit its settings. Most are explained on the settings editor. For the "Library" setting, you must click "Add", then paste the LibraryLocator of your library. This looks like `library-v1:ProblemX+PR0B`. (It's in the URL when you go to the library in studio). Hit Save.
5. Click "Update now with latest components from the library".
6. Optional: Click "View >" to preview all the blocks found in the library.
8. Hit "Publish", then go to the LMS. Browse into the course, and you should see that the block is replaced by random blocks from the library. To test the randomization, click the lowest "STAFF DEBUG INFO" link underneath the module, then click "Delete Student State", then refresh the page.

**Author concerns:**

1. LibraryContent is implemented as an XModule - should likely to be implemented as XBlock, though it's unclear if the necessary infrastructure is yet implemented in the runtime[s].
2. Tests assert against displayed text -- will break if (when) text changed; see inline notes for details on this.
@marcotuts @bradenmacdonald Given the looming deadlines for the MVP, can we go for something simple that we can do within the scope of this PR review for now, and add a story in the backlog about turning it into something more sophisticated down the line?

@marcotuts Can you confirm that having the message in the body is really not an acceptable option for now? If so option 1/deleting would be the only thing we would have time for in the MVP imho. Note that I tend to agree with @bradenmacdonald that not having any information available in the default view seems to leave the user worse off than putting it in the center box like now, but that's your call.