Replace own CKeditor with dedicated app

Use a dedicated app to handle the CKeditor for any text fields, see: #944 If we want to go with this solution, I'll remove the CKeditor JS files from our repo, but I didn't want to do it now so it doesn't "pollute" the commits and the pull request is clearer.

With this solution, it's possible to define specific configs for CK and use them on a per-field basis, like this (we will still need to handle the server side filtering, though):
```python
text = forms.CharField(widget=CKEditorWidget())
reason = forms.CharField(widget=CKEditorWidget(config_name='full_options'))
text_log = forms.CharField(widget=CKEditorWidget(config_name='image_upload'))
```

There is also image upload logic, but I haven't tried it yet. I think @emanuelschuetze mentioned once that this was a wish or planed for the future?

The app can be found here (it's a fork because the original app wasn't mantained anymore):
https://github.com/riklaunim/django-ckeditor