Adding OSSN-0017 - Session-fixation vulnerability in Horizon when using the default signed cookie sessions

OSSN-0017 describes an issue where the default setting in Horizon causes client side cookies to be used.
This allows an attacker who is able to capture a user's cookie to perform any action as that user, even
after that user has logged out.

Related-Bug: #1327425
Change-Id: I74bf8f308227c8adafc719474bec6f8cd1db2601