Always render dates in tooltips when hovering over data points in Burndown charts etc of the Facts application as `YYYY-MM-DD` instead of `YYYY-M-D` by converting the integer to a string and if applicable filling up one character via `padStart()`.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart
`padStart()` support requires Chrome 57, Firefox 48, Edge 15, Opera 44, Safari 10.
Closes T15819