Merge branch '887-fields_for_model-is-not-defined-in-extensibleform' into 'master'

Resolve "'fields_for_model' is not defined in ExtensibleForm"

Closes #887

See merge request !1292

aleksis/core/mixins.py

@@ -14,7 +14,7 @@ from django.db import models
 from django.db.models import JSONField, QuerySet
 from django.db.models.fields import CharField, TextField
 from django.forms.forms import BaseForm
-from django.forms.models import ModelForm, ModelFormMetaclass
+from django.forms.models import ModelForm, ModelFormMetaclass, fields_for_model
 from django.http import HttpResponse
 from django.utils.functional import classproperty, lazy
 from django.utils.translation import gettext as _