10. As an item, I want my estimated next purchase date to be computed at the time my purchase is recorded in the database

[stacietaylorcima]

AC:

• When a purchase is recorded, the estimated number of days until the next purchase date should be calculated and recorded in the database

The following script can be used to calculate the estimated next purchase date: https://gist.github.com/segdeha/a45ef8d6d7f094f4038a2318a9d60051