Graceful handling of private wiki

[garrett@…]

Some of our wikis are private, so we remove all permissions for anonymous. A user's first time visit is met with Error: Forbidden and a note that WIKI_VIEW privileges are required.

A more graceful approach in this case would be to check if the current user is anonymous, and if so, prompt him/her to sign in first.

As a work around, we've re-implemented error.html to check if the error a "Forbidden" and then display a message "This site requires you to login" message with a link to the login page.

A slightly revamped error.html could be used to handle this case more gracefully.

[Emmanuel Blot]

Duplicate of #6439 (with a simple alternative for private installations w/ Apache server)