[fix] seperate toolbar in placeholder field and html field

When using a custom toolbar (by overwriting ``CKEDITOR_SETTINGS['toolbar']``) you cannot use a different toolbar for the placeholder plugin and the ``HTMLField`` widget (despite being prepared [here](https://github.com/divio/djangocms-text-ckeditor/blob/master/djangocms_text_ckeditor/static/js/cms.ckeditor.js#L30-L43) and implemented [here](https://github.com/divio/djangocms-text-ckeditor/blob/master/djangocms_text_ckeditor/templates/cms/plugins/widgets/ckeditor.html#L44-L45) (which will never be considered since ``toolbar`` from ``settings`` will always be used first.)

This fixes this, using ``CKEDITOR_SETTINGS['toolbar_htmlfield']`` instead of ``CKEDITOR_SETTINGS['toolbar']`` when it's not a placeholder and the setting is available.