Fix #122 -- Allow users to re-send order links (#197)

If the provided mail address has not ordered anything, there will still
be a mail generated and sent to an invalid mail address, to avoid
obvious timing attacks to determine active users.

src/pretix/presale/forms/user.py

@@ -0,0 +1,7 @@
+from django import forms
+
+from django.utils.translation import ugettext_lazy as _
+
+
+class ResendLinkForm(forms.Form):
+    email = forms.EmailField(label=_('E-mail'))