MediaWiki Installer: Sidebar group separator should look like in Vector's sidebar
completed by: m4tx
mentors: Bartosz Dziewoński
Fix https://bugzilla.wikimedia.org/show_bug.cgi?id=37362 . Please read the bug report's description and comments to learn what needs to be done. Also see https://www.mediawiki.org/wiki/Skin:Vector for more information about the Vector skin.
Students are required to read Wikimedia's general instructions at https://www.mediawiki.org/wiki/Google_Code-in_2014#Instructions_for_GCI_students first.