comp AP gateway: signature checking, caching and threads management: - HTTP signature is checked for incoming messages - AP actor can now be followed using pubsub subscription. When following is accepted, the node is cached - replies to posts are put in cached pubsub comment nodes, with a `comments_max_depth` option to limit the number of comment nodes for a root message (documentation will come to explain this). ticket 364

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