MediaWiki theme: Use disabled color variable for disabled label
MediaWiki theme: Use disabled color variable for disabled label

Replace custom color value with disabled color variable in order to
have both, input value color and label of disabled textInputWidget
unified.

Change-Id: Ie970625a400942e132bb45a2e34f2884163b66dc

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