Allow voucher code to be passed in via query param

Voucher form now accepts a "voucher" query string parameter
with the value to be pre-populated when the form is rendered.

vendor/registrasion/registrasion/views.py

@@ -650,7 +650,12 @@ def _handle_voucher(request, prefix):
     ''' Handles a voucher form in the given request. Returns the voucher
     form instance, and whether the voucher code was handled. '''
 
-    voucher_form = forms.VoucherForm(request.POST or None, prefix=prefix)
+    initial = {}
+    if request.GET:
+        initial = request.GET.copy()
+
+    voucher_form = forms.VoucherForm(request.POST or None, prefix=prefix,
+        initial=initial)
     current_cart = CartController.for_user(request.user)
 
     if (voucher_form.is_valid() and