mediawiki.special.vforms: Information boxes' styles

Only styles appropriate for errorbox were defined, but they were
applied to all boxes - contrary to the comment above, Vector doesn't
have styles for them (they are shared for all skins, and they are
awful), and they were anyway overriden by these due to selector
specificity.

Added styles for successbox and warningbox with composition on the
color wheel very similar to errorbox one, simply rotated and scaled.
Removed misleading comment.

Change-Id: I68d2c9b98a9166f5781fcded0318cca52ed6f2e6