On-the-fly search indexing is now supported for the Elasticsearch search

backend. When the option is enabled (from the search settings

administration form), the search index will be updated whenever review

requests are updated or deleted.
Additionally, the default value of the Whoosh search index has been

renamed to "search-index", which was the original default; it was

accidentally changed to "search_index" in a previous change.

Ran unit tests.