Replace $wgSpecialPageGroups with SpecialPage::getGroupName()

The global was deprecated in 1.21 and kept for BC.
Translate is currently compatible with 1.22 and later.

Change-Id: I306e41c1a9e26dffdf1bcdb89ecbd036daf53ac9
Merge "Replace $wgSpecialPageGroups with SpecialPage::getGroupName()"