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 looks like with `is not None` this branch was always triggered, as `new_def_data` was always at least an empty dict:

    partitioned_fields = self.partition_fields_by_scope(block_type, fields)  # returns defaultdict(dict)
    new_def_data = partitioned_fields.get(Scope.content, {})

So, was it an expected behavior to always call `update_definition_from_data`?