Add conversations API (#8832)

* Add conversations API

* Add web UI for conversations

* Add test for conversations API

* Add tests for ConversationAccount

* Improve web UI

* Rename ConversationAccount to AccountConversation

* Remove conversations on block and mute

* Change last_status_id to be a denormalization of status_ids

* Add optimistic locking

app/lib/inline_renderer.rb

@@ -13,6 +13,8 @@ class InlineRenderer
       serializer = REST::StatusSerializer
     when :notification
       serializer = REST::NotificationSerializer
+    when :conversation
+      serializer = REST::ConversationSerializer
     else
       return
     end