Introduce custom field for richtext fields

[slomo]

A model field for richtext could bundle up the following settings:

• clean non-allowed html from input, or output or both (using bleach)
• set ck-editor as widget
• idea: mark loaded strings as safe
• idea: meinBerlin team is wrapping contents with a div (assigning a common) class. This is currently happening in the template. We could also do it in the ModelField itself.