Add custom express types

We add a custom User object into the request object. To be able to use
that object in a strongly-typed situation, we need to tell TypeScript
that it exists.

Signed-off-by: David Mehren <git@herrmehren.de>

types/express/index.d.ts

@@ -0,0 +1,13 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { User} from '../../src/users/user.entity';
+
+declare module 'express' {
+  export interface Request {
+    user?: User;
+  }
+}