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.
@dmitchell Ah, I didn't actually realize that `block_id` was only unique when paired with `block_type`. That explains a lot, haha. Thanks for catching that. I will fix.

The reason I iterate over children rather than `selected` here is exactly that - `get_block` requires the block type and block_id, but I was only storing the ID. When I change this so that the block stores both, I will be able to get this to just iterate over `selected`, but it will mean that the block will have to construct a `UsageKey` using `self.location.course_key.make_usage_key()` - which I think may be frowned upon in an XBlock/XModule context?