mw-ui-input for input field at Special:PagePreparation

The mw-ui-input class has been added for the input field used at
Special:PagePreparation to improve the look of the input field.

Change-Id: Ifc25da371e35829912b327f32a9e59adc80dbeae
Merge "mw-ui-input for input field at Special:PagePreparation"