mediawiki.ui: Suppress default webkit-appearance of buttons from iOS

On iOS, buttons made with <input type="submit"> (<button> is not affected)
have a native webkit-appearance that, unlike the desktop equivalent, is
not removed when properties like display, border and background are set.

It remains applied causing it to render awkwardly. Manually reset this
property to remove the default styling.

Bug: 64264
Change-Id: I65525e79503937fd42d4df01a1ce63936f4f92e4
Merge "mediawiki.ui: Suppress default webkit-appearance of buttons from iOS"