Performance: source unit card preview block types and counts from search index

[pomegranited]

Follow up to #1618, cf #1774 (comment)

1. Update search index for containers to include a list of child block usage keys (both published and draft).
2. Ensure the container search index is updated when changes to the container's child list are made.
3. Update frontend to use this data instead of hitting the rest api.

Out of scope:

• Reorder components within unit #1621
• Remove component from unit #1622
• Delete component within unit #1623
• Rename component within unit #1626
• Delete component that's used in a unit #1685

[pomegranited]

@jmakowski1123 @sdaitzman @edschema This is ready for AC testing on the sandbox

There should be no real visible change to the Unit cards with this PR -- we're just using the search index instead of platform APIs to provide the data now.

[edschema]

@pomegranited Is the change to using search index related to performance re load time for unit card previews? The cards are loading very quickly for me, not the ~2seconds referenced by @rpenido. If my understanding is correct, we can mark this done.