Extend RichtTextFields to auto clean html

[kleingeist]

Currently Uploading|RichTextFields are cleaned from unallowed html by using the models save() method [1].
As this is prone for errors because devs forget to call the clean method manually we should extend Uploading|RichTextFields to auto clean their html.
This could be achieved by using a custom field that overwrites the pre_save method [2] as described by [3].

[1] https://github.com/liqd/a4-meinberlin/blob/master/meinberlin/apps/ideas/models.py#L44
[2] https://docs.djangoproject.com/en/1.11/ref/models/fields/#django.db.models.Field.pre_save
[3] https://docs.djangoproject.com/en/1.11/howto/custom-model-fields/#preprocessing-values-before-saving