Charts: Rotate x-axis labels for better readability

Summary:
Rotation is a fundamental movement pattern in our every-day activities.
Whether we play Spin The Bottle at our birthday, change our keyboard layout regularly, or do a sumersault to roll back into bed after our hair of the dog, we all regularly rely on rotation.
Thus also apply this principle to chart labels.

All credits go to http://www.d3noob.org/2013/01/how-to-rotate-text-labels-for-x-axis-of.html

Closes T16013

Test Plan:

1. Go to /project/reports/1/ after having constant task activity within that project tag for a while
2. Try different viewport widths
3. Take a deep breath and admit that the x-axis labels are more readable than before

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16013

Differential Revision: https://we.phorge.it/D25908