comp AP gateway: publish, (un)subscribe/(un)follow, public subscription/following/followers: this patch implements those major features: - `publish` is implemented on virtual pubsub service, thus XMPP entities can now publish to AP using this service - replies to XMPP items are managed - `inReplyTo` is filled when converting XMPP items to AP objects - `follow` and `unfollow` (actually an `undo` activity) are implemented and mapped to XMPP's (un)subscribe. On subscription, AP actor's `outbox` collection is converted to XMPP and put in cache. Subscriptions are always public. - `following` and `followers` collections are mapped to XMPP's Public Pubsub Subscription (which should be XEP-0465, but the XEP is not yet published at the time of commit), in both directions. - new helper methods to check if an URL is local and to get JID from actor ID doc will follow to explain behaviour rel 365

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