Add metrics for item count and file size sums

- Added "artistCount", "albumCount", "songCount", and "videoCount" to Status API, along with associated data model count functions
- Added "songTotalSize" and "videoTotalSize" (in bytes) to Status API, along with associated data model sum functions

Subsonic and MPD have these, and they're fast using aggregate functions, so why not?  This will (probably) be my last pull request today, haha.