component AP gateway: AP delete activity => message retract: handle retractation of messages. As it is not possible to know from the AP item alone if we need to to a message retractation (XEP-0424) or a pubsub retractation (XEP-0060), we now cache sent message, and decide which method to use according to how the item is cached (i.e. in message history or in pubsub cache). rel 367

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)