BUG Unable to use 'Created', 'LastUpdated' or 'ClassName' in $searchable_fields

DO NOT MERGE: needs reviewing

If you define 'Created', 'LastUpdated' or 'ClassName' in a model's `$searchable_fields`, an error occurs: "Call to a member function scaffoldSearchField() on a non-object in .../DataObject.php on line 1973".

This is because `relObject()`, and in turn `dbObject()` return null for any of those fields. The reason for this is that `DataObject->db()` gets its array of database fields from the Config manifest with:

`Config::inst()->get($class, 'db', Config::INHERITED);`.

This doesn't include those fields in the returned data.

Changing the above line to instead use `self::database_fields($class);` fixes the issue, but adds 'Created', 'LastUpdated' and 'ClassName' to the scaffolded form fields.

Proposed solution is attached, but will need reviewing by someone with a better understanding of the implications of this change. `database_fields()` calls `custom_database_fields()` which appears to add has_one relationships to the array using dot notation, so that's an extra consideration.