* feat: store content.child_usage_keys in Container search document
Stores the draft children + published children (if applicable)
Related fixes:
* fix: lib_api.get_container does not take a "user" arg
* refactor: fetch_customizable_fields_from_container does not need a "user" arg
* refactor: moves tags_count into LibraryItem
because anything that appears in a library may be tagged.
* refactor: remove get_container_from_key from public API
API users must use get_container and ContainerMetadata.
* refactor: made set_library_item_collections take an entity_key string instead of
a PublishableEntity instance, so we don't need to fetch a Container object to call it.
* refactor: changed ContainerLink.update_or_create to take the container PK
instead of a Container object, since this is enough.
Added container_pk to ContainerMetadata to support this.
5b3caa93e2