Fixing width of icon-only-buttons in Vector skin

MediaWiki uses a custom stylesheet for buttons in Vector skin. This
stylesheet, however, breaks the jQuery UI icon-only buttons by setting
their width to "auto". This patch will re-set the width to 2.2em for
single-icon buttons and to 3.4em for 2-icon buttons.

Bug: 48593
Change-Id: I650d0e211cb4a1ce47c3d2246a3f266316afbb54