Fix email token input

Following using an integer field instead of a character field, a bug
with the token input was introduced.

Addresses the following:

- Adds code to ensure the string of the token is 6 characters long by
  padding it with leading zeros.

Contributes towards: #107

apps/users/views.py

@@ -344,7 +344,7 @@ def populate_message(self, challenge_passes, token_within_expiry):
 
     def form_valid(self, form):
         super().form_valid(form)
-        token_returned = str(form.cleaned_data['token'])
+        token_returned = str(form.cleaned_data['token']).zfill(6)
         challenge_passes = self.does_challenge_pass(token_returned)
         email_token = self.get_email_token()
         if challenge_passes and email_token.is_challenge_within_expiry: