Item - Load Item and all its children

[mariusor]

See #2

Since our basic Item is an ActivityPub Article object, and it doesn't have an outbox itself, this will be tricky to implement using strictly the AP spec.

An idea would be to have a way of filtering items by Audience - and expose it for Article items.

[Updated] All AP Objects have a Replies collection. I populate this collection with the URIs of those items.

[mariusor]

For the moment I solved this by using a separate request from the frontend to the API which filters by inReplyTo.