Added new setting 'CKEDITOR_RESTRICT_BY_USER_ATTR'

Hi,

I needed to implement a feature to restrict access based on a user attribute 
This is useful when you have groups of users and want users from the same group to share the uploads.

If we have the method `my_group_tag` in the `User` class we can define the following in the `settings.py`

```python
CKEDITOR_RESTRICT_BY_USER_ATTR = 'my_group_tag'
```