Calculate vote average using weighting by date

[whomwah]

Currently the average vote for a track is calculated in the standard way, taking all the votes and dividing them by the count.

This change adds weighting to the calculation so the longer ago a vote is cast the less important it is to the average.

Based on todays date (2020-02-06) here's some weighted votes based on the original and the date they were voted:

{ vote: 90, at: '2020-02-05T17:08:27.000Z', weighted: 90 },
{ vote: 80, at: '2019-08-04T17:08:27.000Z', weighted: 70 },
{ vote: 10, at: '2019-02-03T17:08:27.000Z', weighted: 30 },
{ vote: 10, at: '2018-08-02T17:08:27.000Z', weighted: 40 }

Remember the scores are stored as 0-100 but are displayed as -5 to +5 so a score of 40 = -1