Add BestBetsChecker to look up best bets

This fetches best bets from the metasearch index, and merges multiple
best bets appropriately.
Tell test helper about metasearch helper
Raise a clearer error for invalid index names
Look up and use best + worst bets at search time

Add metasearch index to app.  Pass it through UnifiedSearcher to
UnifiedSearchBuilder.  Use a BestBetsChecker to look up bets from
UnifiedSearchBuilder, and build a query which obeys the bets if there
are any.