Add linear regression for weight in Statistics page

[NelsonAPenn]

I checked for duplicates and did not find any. This is a similar request to #889 but is not the same.

It would be helpful to show or have the option to show the linear regression of weight over the selected period in the Statistics page.

For example, the user could select "Weight-- 1 Month" and see the trendline and slope of all weight measurements over that period (suggested units for slope would be the user-selected weight unit / week). This would be another extremely useful tool to show whether your rate of weight loss or gain is on track (e.g. 0.25 lbs / week bulk, 1 lb / week cut).

I imagine the same concept would apply equally for any of the "Body Stats".