Make configurable, if the login form will be displayed to unauthenticated users on access deny pages

Currently, if an unauthenticated user hits an "access deny" page, the login form is displayed. This feature is hardcoded and this is inconvenient in some cases. For example, when DokuWiki is configured to use an external authentication (like Shibboleth), the login form should not be displayed, because it will confuse the users.

This pull request suggests adding a configuration directive to switch on/off the login form display. The login form ill be displayed by default. I'm not sure, if the name of the directive complies to the DokuWiki conventions, but I can change it.