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

sat/plugins/plugin_comp_ap_gateway/constants.py

@@ -24,15 +24,18 @@ TYPE_ACTOR = "actor"
 TYPE_INBOX = "inbox"
 TYPE_SHARED_INBOX = "shared_inbox"
 TYPE_OUTBOX = "outbox"
+TYPE_FOLLOWERS = "followers"
+TYPE_FOLLOWING = "following"
 TYPE_ITEM = "item"
 MEDIA_TYPE_AP = "application/activity+json"
+NS_AP_PUBLIC = "https://www.w3.org/ns/activitystreams#Public"
 # mapping from AP metadata to microblog data
 AP_MB_MAP = {
     "content": "content_xhtml",
 
 }
 AP_REQUEST_TYPES = {
-    "GET": {"actor", "outbox"},
+    "GET": {TYPE_ACTOR, TYPE_OUTBOX, TYPE_FOLLOWERS, TYPE_FOLLOWING},
     "POST": {"inbox"},
 }
 # headers to check for signature