Empêche les conflits username-clipper

Le formulaire de création d'utilisateur empêche de choisir un username de
moins de 8 caractères.

Fixes #57

gestioncof/forms.py

@@ -10,6 +10,7 @@ from django.contrib.auth.models import User
 from django.forms.widgets import RadioSelect, CheckboxSelectMultiple
 from django.forms.formsets import BaseFormSet, formset_factory
 from django.db.models import Max
+from django.core.validators import MinLengthValidator
 
 from gestioncof.models import CofProfile, EventCommentValue, \
     CalendarSubscription
@@ -201,6 +202,7 @@ class RegistrationUserForm(forms.ModelForm):
     def __init__(self, *args, **kw):
         super(RegistrationUserForm, self).__init__(*args, **kw)
         self.fields['username'].help_text = ""
+        self.fields['username'].validators = [MinLengthValidator(9)]
 
     class Meta:
         model = User