component AP gateway: when a repeated blog post is retracted, it converted to suitable Activity: when a retract item is received, the former item is now retrieve from cache, and if it was a "repeat" blog post, it is converted to the suitable `Undo` of `Announce` activity (instead of the default `Delete` activity). rel 370

these certificates are used for demo purpose only. Those must be the same as in
backend_e2e (they are generated there)