Django-content-settings

* `from content_settings.conf import settings`

* new attributes `on_change` and `on_change_commited`
* pass user object in the preview


* admin can now preview settings on site before applying
* new middleware `middlewares.preivew_on_site`

* help_text is alternative for help attribute
* fix warnings
* using urls is optional

* `SplitByFirstLine` is now in `EachMixin`
* `SplitByFirstLine.split_type` can be a dict with key:cs_type
* `AdminPreviewActionsMixin`
* new attribute `validators_raw`