[prep-release-3.0.12] Bumping version number for 3.0.12-RC2.
[prep-release-3.0.12] Update Changelog for 3.0.12-RC2 release.
Merge branch 'prep-release-3.0.12' of git://github.com/phpbb/phpbb3 into prep-release-3.0.12

* 'prep-release-3.0.12' of git://github.com/phpbb/phpbb3:
  [ticket/11753] Remove ?> from MySQL Upgrader.
  [ticket/11753] Update MySQL upgrader schema data.
  [ticket/11524] Add another isset() to mitigate "Illegal string offset 'limit'"
  [ticket/11752] HTTP -> HTTPs in email/installed.txt