Unable to login if single sign-on is disabled

Whenever I disable the single sign on through the filter

```
add_filter( 'mercator.sso.enabled', '__return_false' );
```

I tracked down the cause of this to be the fact that the COOKIE_DOMAIN isn't defined as including something like

```
$dm_domain = $_SERVER[ 'HTTP_HOST' ];
define( 'COOKIE_DOMAIN', $dm_domain );
```

allows it to work again.