Special page: standard message for no permission

On the special page: removed the overlay guider that tells
anonymouos users that they can't create articles.
Now, if a user that can't create an article goes to the
special page, we throw an error in PHP, which redirects
to a page with an appropriate message.
Better error handling on special page

Added error handling if we don't get the proper
URL parameters, and added UserNotLoggerIn error
for anon users that try to access the special page.
Remove special page from list of special pages

Since the special page doesn't work without URL parameters
for the article to create and for returning to the previous
page, there's no sense in having it publicly available
from the list of special pages.
ext.articlecreationhelp: A bit of cleanup, esp. doc.
ArticleCreationHelp.php: minor cleanup
ext.articlecreationhelp.js: cleanup & better separation of concerns