Library Content - Filter library content by CAPA problem type

**Background**: This PR contains the LibraryContent XBlock, which allows to display library content in a course.
**JIRA ticket**: [SOL-122](https://openedx.atlassian.net/browse/SOL-122)
**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/6155 (probably will require a rebase after 6155 merged)
**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/24
**Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.

**Author concerns**: see inline comments

**Testing instructions**:
1. Create a library with a couple of Problem XBlocks of different types (e.g. "Checkboxes", "Dropdown", "Multiple Choice")
2. Add Library Content XBlock to a course.
3. Edit Library Content settings. By default "Problem Type" should be set to "Any Type", LMS view should display problems of any type.
4. Set "Problem Type" to one of the problem types in the library (e.g. "Checkbox Group", "Select Option" or "Choice Group"). Please note that there are no one-to-one correspondense between names of problem *templates* in Studio and actual names of problem types. Save, publish, navigate to LMS as a student and ensure only Problems of selected type are shown.
5. Set "Problem Type" to any problem type **not** in the library (e.g. any except "Checkbox Group", "Select Option" or "Choice Group"). Save, publish, navigate to LMS as a student and ensure no XBlocks are shown.
@dmitchell thanks for suggestion. `capa_type` is indeed xml tag names, so this is a very low-level approach, but could do the trick actually. Until CAPA problem format changes, but it doesn't look like it happens too often.