Fix ambiguious routes

[marksteward]

Change these routes so voucher_code and flow can't clash with existing routes:

@admin.route("/product_views/<int:view_id>/voucher/<string:voucher_code>")
@tickets.route("/tickets/voucher/<voucher_code>")

@tickets.route("/tickets/<flow>", methods=["GET", "POST"])

cfp_type isn't user editable.

[russss]

The issue I hit here is that if you create a ProductView called "voucher" that gets built as /tickets/voucher which conflicts with the other routes under /tickets/