[Stats] File Downloads: capture new endpoint response data

[ScoutHarris]

With the changes noted in this P2, the following has been added to the file-downloads endpoint response:

• relative_url
• download_url

WPKit:

• StatsFileDownloadsTimeIntervalData - update StatsFileDownload and init parsing.
• Update testFetchDownloadsData and stats-file-downloads.json.

WPiOS:

• Update Core Data, specifically:
  • FileDownloadsStatsRecordValue+CoreDataClass
  • FileDownloadsStatsRecordValue+CoreDataProperties
• Update FileDownloadsStatsRecordValueTests.

These URLs aren't used in WPiOS, but will be needed if we decide to link to the files as Calypso does.