Add tooltips for numbers in workboard column headers
Summary:
The mystery has been resolved. Brave mouse pointers can now finally reveal an explanation how long forgotten generations of product managers wanted to plan and groom and such.
Closes T16106
Test Plan:
* Establish backlog bankruptcy.
* Go to a project workboard with columns and column limits and tasks.
* Hover over the numbers in the column headers.
* Switch on and off `/config/edit/maniphest.points/` and experience an exciting plot twist by repeating the previous step.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16106
Differential Revision: https://we.phorge.it/D26079