skin: Revert font-weight on successbox and move to preferences

Removed the font-weight: bold from the css rule of the error,
warning and successboxes and placed this css declaration in
mediawiki.special.preferences.css under a new class
"mw-sp-pref-successbox".

Follows-up 44e64fb.

Bug: 48197
Change-Id: I1a3d774c2daef158176479fba7828b383a195bb9
Merge "skin: Revert font-weight on successbox and move to preferences"