Django-content-settings

* internal constants refactoring to Enums.
* validation changes on the list view using `content_settings_context`
* Admin: fix delete link

* new attributes `on_change` and `on_change_commited`
* pass user object in the preview
* `from content_settings.conf import settings`

Add preview on site middleware to be able to see the changes live, before applying those for all users.

* 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`

* Fixing a lot of bugs with admin of content settings. Such as preivew, search, tags filters and cetera
* fixing dynamic work with local changes in content settings
* fix help text for user defined types
* if variable constant or removed from code - admin is not allowed to see it in django admin
* new type `DjangoModelTemplateHTML`
* fix issue with \r\n in tags field for user defined types
* remove attribute `empty_is_none`, we have `EmptyNoneMixin` mixin instead
* `validators` can be tuple and list now
* `EachMixin` now works only with expected types.
* new class for `EachMixin` - `each.Values` - convers all values in the dict
* fixing bugs with lazy object
* for `CallToPythonMixin`, `preview_validators` and `admin_preview_call` are removed. Now, all of the validators, that are instance of `call_validator` will be shows in preview and settings call will be always shown if more than one validators are for preview
* the change above requires massive refactoring for preview of template/call types
* if `SimpleCallTemplate` doesn't have a required arguments and doesn't have validators - the system uses one validator `call_validator()`
* `DjangoTemplateHTML(HTMLMixin, DjangoTemplate)` -> `DjangoTemplateHTML(HTMLMixin, DjangoTemplateNoArgs)`
* * `SplitByFirstLine.split_key_validator_failed`
* fix preview for `each.Keys`