edit.html: we were only replacing a single instance of &, not all of them.

This meant if you were editing a row with url=...?a=b&c=d&e=f, the &e=f
would not be escaped properly.
diff --git a/edit.html b/edit.html
index b70e6d2..2d0f03d 100644
--- a/edit.html
+++ b/edit.html
@@ -49,7 +49,7 @@
   // we could use encodeURIComponent() here, but it's too fancy.  Let's
   // *only* encode & characters so the resulting URL looks as nice as
   // possible.
-  return s.replace('&', '%26');
+  return s.replace(/&/g, '%26');
 }