[custom fields] Call setObject before shouldEnableForRole

[marcelofabri]

By calling setObject before shouldEnableForRole, we allow methods like shouldAppearInListView in a PhabricatorCustomField subclass to access getObject() -- and more importantly, getObject()->getRepository().

This will allow a custom field to use information about the current repository to decide whether it should be shown or hidden.

If the field is indeed disabled, the unset later should clear up everything from $field, including its $object reference.