Replace _ in username to avoid markdown issues

When gifting a user gold, it displays their username using markdown,
so any underscores need to be replaced first to avoid having part of
a username between them interpreted as italics.

Fixes issue #714

r2/r2/lib/pages/pages.py

@@ -1935,7 +1935,8 @@ class GoldPayment(Templated):
                 if not clone_template:
                     summary = format % dict(
                         amount=Score.somethings(months, "month"),
-                        recipient=recipient and recipient.name,
+                        recipient=recipient and
+                                  recipient.name.replace('_', '_'),
                     )
                 else:
                     # leave the replacements to javascript