refactor: use a base dto class

This gives all dto classes a common super class for usage of the type system.

Signed-off-by: Philip Molares <philip.molares@udo.edu>

src/utils/base.dto..ts

@@ -0,0 +1,10 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+/**
+ * The base class for all DTOs.
+ */
+export abstract class BaseDto {}