Hide signup/login/logout links when they would not work

Hide signup/login/logout links when they would not work

Immutable session providers do not support login and logout;
these pages would just show an error when such a session provider
is handling the authentication of the request. Depending on what
authentication providers are configured, the wiki might not
support account creation. Showing the links is unhelpful if
they would just show an obscure error message.

(OTOH don't try to hide the link when the reason it is not
usable depends on the user; specifically, don't check
AuthManager::checkAccountCreatePermissions. An error message
is more informative in that case than a missing link.)

Also improve how the affected special pages behave if the user
manages to get there anyway.

Change-Id: Ic0ad237259797a8d471bdabc57a4bd0ffe8fa33b