Bug 26102: Prevent XSS when To.json is used: subscription-add.tt

Bug 26102: Prevent XSS when To.json is used: subscription-add.tt

Test the process of adding a subscription, entering both a valid vendor
ID and a non-existent vendor ID. The non-existent vendor ID should
trigger a validation alert.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 583aad8e48790443a14ac4b7dfe85fa1bdeb91a2)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>