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

You'll find install instructions in documentation (see doc/installation.rst).

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