Chats: Add cascading delete on both referenced users.

Also remove the now-superfluous join in the chat controller,
which was only used to filter out these cases.

priv/repo/migrations/20200831142509_chat_constraints.exs

@@ -0,0 +1,22 @@
+defmodule Pleroma.Repo.Migrations.ChatConstraints do
+  use Ecto.Migration
+
+  def change do
+    remove_orphans = """
+    delete from chats where not exists(select id from users where ap_id = chats.recipient);
+    """
+
+    execute(remove_orphans)
+
+    drop(constraint(:chats, "chats_user_id_fkey"))
+
+    alter table(:chats) do
+      modify(:user_id, references(:users, type: :uuid, on_delete: :delete_all))
+
+      modify(
+        :recipient,
+        references(:users, column: :ap_id, type: :string, on_delete: :delete_all)
+      )
+    end
+  end
+end