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
0114769fa4