Make changes to the Mongo modulestore format for asset metadata.
Move the asset types under a top-level 'assets' dict to allow for
future arbitrary asset types.
Change 'basename' to 'pathname', since the attr is for storing the path.
Merge pull request #6166 from edx/jeskew/modulestore_asset_md_format_changes

Make changes to the Mongo modulestore format for asset metadata.

https://openedx.atlassian.net/browse/PLAT-301