PopupWidget: Remove box-shadow rule that generates invisible shadow

The shadow is not visible because of the 'overflow: hidden;' on
.oo-ui-popupWidget-popup. MediaWiki theme is not supposed to have it,
Apex was creating a second, visible shadow on .oo-ui-popupWidget-popup
itself.

Change-Id: Icadc4fb8b4705d69e950c5d6829f19f761628f7e
MediaWiki theme: Improve search widget styling

Bug: T78034
Bug: T78035
Change-Id: I74cd2a9cca28f8c083ca44197402f4170b8964f0
Merge "PopupWidget: Remove box-shadow rule that generates invisible shadow"
Merge "MediaWiki theme: Improve search widget styling"