Summary:
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 its length is only one character, prepend a zero.
Closes T15819
Test Plan:
* Enable the Facts application, go to the Reports of a Project with task changes over time, hover over data points and read the tooltip.
* Check Console of web browser's developer tools for no errors.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15819
Differential Revision: https://we.phorge.it/D25630