MediaWiki theme: Use variable for disabled ProgressBar
MediaWiki theme: Use variable for disabled ProgressBar

Replacing non-guideline opacity value with Less variable for
disabled ProgressBar in alignment with disabled widgets elsewhere.

Change-Id: I6e47cb6eca01db95f53ad3ffaa0f7a4e6c555ffa

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui