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.
In general, lean towards not using anything out of `self.runtime` that's not accessible via a `self.runtime.service(..)` call, as those are likely to be attributes that we're going to be getting rid of.

For rendering in particular, you can just use the Mako library directly, rather than using it via `self.runtime.render_template`.