Added configuration option to only allow OpenID logins.

Added configuration option to only allow OpenID logins.

If $config['site']['openidonly'] is set to true:
* the Login/Register pages will be removed from the navigation;
* directly accesses to the Login/Register pages will redirect to the
  OpenID login page;
* most links to the Login/Register pages will link to the OpenID login
  page instead.

The user will still need to set a password to access the API and RSS
feeds.