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.
@dmitchell it makes some sense, but I would suggest creating separate method for that, rather than adding another mode to `_verify_modulestore_support`. It would also make a terrible naming: `verify` checks for something, not `find`s. 

So, my suggestion is that it's possible to do so, but it would better be a separate method (e.g. `_find_modulestore_with_method` or something). And since it is not directly required by anything in this PR, it should be done separately.

Are you ok with that?