Adding historic dates flag to search results

Adds an "historic" item to search.json
• adds an empty string if the service's latest quarter is the same as the global latest quarter
• adds the service's latest quarter as a string if it's less than the global latest quarter

In rendering, if "historic" is not an empty string, the value is wrapped in a span with title attribute (for example see https://www.gov.uk/performance/transactions-explorer/high-volume-services/by-transactions-per-year/descending)

Related tests have been amended to pass, but if anyone wants to pair up to write a specific test here, then shout - as I know not what I do.