FIX: some participant values were removed if participant was edited

Some participant fields were not included in the form template and
thus every time the form was saved those values were emptied.
We exclude those fileds now within the form class.

dav_events/forms/participant.py

@@ -7,7 +7,9 @@ from ..models import Participant
 class ParticipantForm(forms.ModelForm):
     class Meta:
         model = Participant
-        exclude = ['event', 'created_at', 'position', 'purge_at']
+        exclude = ['event', 'created_at', 'position',
+                   'privacy_policy', 'privacy_policy_accepted',
+                   'paid', 'purge_at']
         widgets = {
             'emergency_contact': forms.Textarea(attrs={'rows': 4}),
             'experience': forms.Textarea(attrs={'rows': 5}),