Migrations: Add chat_id, seen index to ChatMessageReferences

This ensures fast count of unseen messages

priv/repo/migrations/20200603122732_add_seen_index_to_chat_message_references.exs

@@ -0,0 +1,12 @@
+defmodule Pleroma.Repo.Migrations.AddSeenIndexToChatMessageReferences do
+  use Ecto.Migration
+
+  def change do
+    create(
+      index(:chat_message_references, [:chat_id],
+        where: "seen = false",
+        name: "unseen_messages_count_index"
+      )
+    )
+  end
+end