Implement course-wide asset paging methods.
Add SortedCollection class to help sort lists efficiently.
https://openedx.atlassian.net/browse/PLAT-74
Implement copy_all_asset_metadata for Old Mongo->Old Mongo.
Add tests for copying all asset metadata.
https://openedx.atlassian.net/browse/PLAT-74
Changes for PR review: More compact tests, enumeration for metadata
sort order, etc.
https://openedx.atlassian.net/browse/PLAT-74
Merge pull request #5819 from edx/jeskew/asset_mongo_coursewide

Implement course-wide asset paging methods.