Re-add support for "extra inboxes"

This allow to start posting public activities to other instances
without follower.

activitypub.py

@@ -19,6 +19,7 @@ from config import ME
 from config import USER_AGENT
 from config import USERNAME
 from config import MEDIA_CACHE
+from config import EXTRA_INBOXES
 from utils.media import Kind
 from little_boxes import activitypub as ap
 from little_boxes import strtobool
@@ -65,6 +66,9 @@ class MicroblogPubBackend(Backend):
         """Setup a custom user agent."""
         return USER_AGENT
 
+    def extra_inboxes(self) -> List[str]:
+        return EXTRA_INBOXES
+
     def base_url(self) -> str:
         """Base URL config."""
         return BASE_URL