implement automatic creation of mailbox names

- create mailbox name when saving new mailboxes in admin
- add MailboxManager that implement get_next_mailbox_name as manager for
  Mailbox

gnuviechadmin/managemails/admin.py

@@ -67,6 +67,8 @@ class MailboxCreationForm(forms.ModelForm):
 
         """
         mailbox = super(MailboxCreationForm, self).save(commit=False)
+        mailbox.username = Mailbox.objects.get_next_mailbox_name(
+            mailbox.osuser)
         mailbox.set_password(self.cleaned_data['password1'])
         if commit:
             mailbox.save()
@@ -118,7 +120,7 @@ class MailboxAdmin(ActivationChangeMixin, admin.ModelAdmin):
     add_fieldsets = (
         (None, {
             'classes': ('wide',),
-            'fields': ('osuser', 'username', 'password1', 'password2')}),
+            'fields': ('osuser', 'password1', 'password2')}),
     )
     search_fields = ('username',)
     ordering = ('username',)