A previous patch introduced support for Elasticsearch instead of Whoosh

as an alternative search engine backend. Now, both Whoosh and

Elasticsearch are provided as SearchBackends and we produce the search

configuration form dynamically based on what search backends are

available.
We now use a registry to manage these backends. Extensions can now

register additional search backends (such as for Solr or other

Haystack-supported backends) through this interface.
The search settings have been moved out of the general settings into its

own form as the logic just for the form generation was getting rather

complicated.

Ran unit tests.
Verified the settings form worked correctly.