Modulestore Support for Content libraries (Part of SOL-1, SOL-2, SOL-3)

@cpennington @dmitchell @doctoryes @antoviaque @explorerleslie @cahrens

**Background**: This is the technical foundation required for [Content Libraries](https://openedx.atlassian.net/wiki/display/SOL/Content+Libraries). It is considered part of SOL-1, SOL-2, and SOL-3, with the corresponding UI changes separated out into #6046.

**Discussion**: Architecture discussed extensively on [the wiki](https://openedx.atlassian.net/wiki/display/SOL/Content+Libraries) and in meetings, then the revised proposal was presented to the [Arch Council](https://openedx.atlassian.net/wiki/display/AC/Architecture+Council) on Oct. 21 and given thumbs up.

**Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.

**Merge deadline**: ASAP - several other PRs are ready and blocking on this.

**Dependencies**: Requires edx/opaque-keys#46, which is already merged.
It's a convenience method and a DRY helper. Often we care if we're requesting a course or a library specifically, but other times we don't care. In the latter case, there would be many places where the code looks like:

```python 
    if isinstance(key, LibraryLocator):
        return modulestore().get_library(key)
    else:
        return modulestore().get_course(key)
```

... hence the helper method.