Add ability to retrieve item stats to api

[tjenkinson]

This can use the playback_history table (#583) to determine:

• when items were watched
• how long items were watched for
• which parts of items were watched
• how many times items were watched
• how many times items were watched in any specific time span

This should be available at a /mediaItems/{id}/stats endpoint and also /mediaItems/stats for global stats.

[tjenkinson]

specific media item

Example url: mediaItems/20/stats?streamChunkPeriod=60&streamChunkStart=0&streamNumChunks=4

• streamChunkPeriod: number of seconds each chunk represents (defaults to something sensible)
• streamChunkStart: chunk number to start at (defaults to 0)
• streamNumChunks: the number of chunks to retrieve (defaults to getting all chunks)
• also vodChunkPeriod etc..

{
    stream: {
        // these totals are the totals this media item has had over all time
        totals: {
            views: 100,
            uniqueViews: 20
        },
        // chunks are from the most recent live stream this media item has had
        chunks: {
            chunkPeriod: 60,
            chunkStart: 0,
            data: [
                {
                    views: 20,
                    uniqueViews: 19,
                    numWatching: 20
                },
                {
                    views: 50,
                    uniqueViews: 1,
                    numWatching: 30
                },
                {
                    views: 30,
                    uniqueViews: 0,
                    numWatching: 90
                }
            ]
        }
    },
    vod: {
        // same as Live except the live format uses the time values were 
        // recorded in the chunks whereas for VOD it can use the actual
        // time positions in the video
        // chunks map to the most recent upload for this media item
    }
}

[tjenkinson]

Implemented /mediaItems/stats/watchingNow
#771