Fix invalid CSS transition value for .jx-scrollbar-handle

Summary:
The W3C CSS validator throws 0 is not a transition value : opacity 0 linear.
0 is not a valid value; likely 0s were meant as in other existing CSS definitions.
See https://developer.mozilla.org/en-US/docs/Web/CSS/transition

Test Plan: Paste standard-page-view.css into https://jigsaw.w3.org/css-validator/ and read some CSS docs.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25923