Drop dead and redundant special page code

* SpecialPage::execute() is not supposed to return anything.
  Warning, this possibly causes the only WikibaseQuery special page to
  fail, please merge https://github.com/wmde/WikibaseQuery/pull/60 too!
* Drop redundant SpecialItemResolver class, all methods and properties
  do exist in the base class. Warning, please have a look at the
  message used in the ...->outputHeader( ... ) call, it changes from
  "<name>-summary" to "wikibase-<name>-summary" now. But none of these
  messages exist for these 3 special pages so it should not make a
  difference.
* Special page names are canonical, never multi-byte, right? No need
  to use Language::lc(), strtolower() is enough.
* Drop unused $subPage property.
* Add some docs.
* Fix wrong visibility level for getGroupName().

Change-Id: Iff0728ccaef50ec41dd7a58a431aee6ceec2a493
Merge "Drop dead and redundant special page code"