Core changes for GlobalBlocking and TorBlock extensions, plus some core refactoring...

Core changes for GlobalBlocking and TorBlock extensions, plus some core refactoring work:
* Instead of saying 'do that' in a permissions error, actually list what the action is (drawn from the right-$1 messages). This isn't perfect - it says you don't have permission to edit pages when
you can't edit a single page, but it's better than 'do that'.
* Refactor out some code from various block files into Block::formatExpiry and Block::parseExpiryInput.
* Don't display 'you cannot edit special pages' when you're trying to execute, or create an account, or something like that.
* New AbortAutoblock hook (for use in TorBlock extension), which allows extensions to cancel autoblocks.