This change standardises the search document "id" to be a meilisearch ID
generated from the usage key, for all types of indexed objects.

This is important for collections so we can locate the collection
document in the search index solely from the data provided by the
LIBRARY_COLLECTION_DELETED event (library_key + collection_key), even if
the collection has been deleted from the database.

* get_library_collection_usage_key and
  searchable_doc_tags_for_collection do not need a Collection object;
  the usage key can be created from the library_key and collection_key.

* updated searchable_doc_for_collection to require the parts of the
  collection usage key + an optional collection. This allows us to
  identify the collection's search document from its usage key without
  requiring an existing Collection object (in case it's been deleted).
  Also removes the edge case for indexing Collections not associated
  with a ContentLibrary -- this won't ever really happen.

from openedx/openedx-learning#229

so that added/removed collections are removed/re-added to component documents.

Special case: When a collection is soft-deleted/restored, we detect this
in the search index and update the collection's component documents
directly, without a CONTENT_OBJECT_ASSOCIATON_CHANGED signal.

typos in code and comments