Split includes/SpecialPage.php

Split includes/SpecialPage.php

Separated base classes for special pages (most of them abstract) and
several actual special pages, which were previously both in the same
file.

Added license headings, @ingroup SpecialPage and updated class doc
comments; no other changes.

Most classes are now in separate files (the list of changes can be
seen by diffing AutoLoader.php), with three exceptions:
* SpecialListAdmins and SpecialListBots (redirects to
  Special:ListUsers with a parameter) joined their friend in
  includes/specials/SpecialListusers.php.
* SpecialMycontributions, SpecialMypage, SpecialMytalk,
  SpecialMyuploads and SpecialAllMyUploads were moved into a new file
  includes/specials/SpecialMyRedirectPages.php – each of them is
  literally ten lines of code and any changes are likely to touch them
  all, separate files seem impractical.
* RedirectSpecialArticle and SpecialRedirectToSpecial are in one file
  with their parent class RedirectSpecialPage (on a side note, I filed
  bug 58215 to rename them to something less silly).

Change-Id: Ida87238401b182924dbe169a6278588bc2fbecfd