Fixed entities wrongly visible on 404

Also ensured header state as expected on 404.
In reference to BookStackApp/website#9

app/helpers.php

@@ -37,6 +37,15 @@ function user()
     return auth()->user() ?: \BookStack\User::getDefault();
 }
 
+/**
+ * Check if current user is a signed in user.
+ * @return bool
+ */
+function signedInUser()
+{
+    return auth()->user() && !auth()->user()->isDefault();
+}
+
 /**
  * Check if the current user has a permission.
  * If an ownable element is passed in the jointPermissions are checked against