Migrate advanced search test to new indexing.

As part of this, pull out elasticsearch integration test code into a
shared module.
Move remaining integration tests to new indexes.

Also:
  * Remove hard-coded list of sitemap indices
  * Move reindexing into the Index class as `populate_from`
Add a method to list indices in a group.
Add a method to Index to get its real name.
Add integration tests for index groups.
Fix broken advanced search tests.
Remove the elasticsearch admin wrapper.

Hooray!
Remove unused stubbing helpers.
Remove old reindexing code.
Remove old backends code.
Remove temporary settings shim.