Facts: Always render YYYY-MM-DD dates with eight digits in chart tooltips

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