mediawiki.ui: Remove white-space: nowrap; for buttons

They are already display: inline-block; to prevent standard text-like
line-breaking; however, buttons with very long text might need
internal line-breaks to display nicely.

Bug: 60660
Change-Id: I095e375c498cd0f92615ec8f42e9ce5257f0213b
Merge "mediawiki.ui: Remove white-space: nowrap; for buttons"