Clean up, comments, remove obsolete mw-ui-formlist

Clean up, comments, remove obsolete mw-ui-formlist

* Remove obsolete ".mw-ui-formlist div label" selector, replaced by
  ".mw-ui-vform > div label".
* Rename mw-ui-block-label to mw-ui-label since it doesn't impose
  block styling.
* Remove overspecified ".mw-ui-vform > div label.mw-ui-checkbox-label"
  selectors, since the bare class applies the same styling.

Also in the CSS for the vertical forms,
* reduce the margin-bottom of #languagelinks
* allow word wrap to break words in the errorbox (bug 46966).

Change-Id: I03bdf3b1c50be732435e10193f004230a6a3bacf