Separate index names into content and auxiliary.

The page-traffic index is an example of an auxiliary index; it doesn't
hold content, but is used outside the main query (in this case, at
indexing time). We'll shortly be adding another such index to hold
best-bets.

Sitemaps should be made only from content indexes.  The search server
needs to know all the allowed names for its validation of index names
when opening indexes.

Conflicts:
	elasticsearch.yml
Add debugger in test & dev.
Fix tests for auxiliary indexes.
Whitespace.
Add `metasearch` index.

Holds information about search such as "best bets",
a mechanism for altering the results returned by
search.
Merge pull request #220 from alphagov/add-metasearch-index

Add metasearch index