feat(backend): handle username always in lowercase

This should make all usernames of new users into lowercase. Usernames are also searched in the DB as lowercase.

Signed-off-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

backend/src/utils/username.ts

@@ -0,0 +1,11 @@
+/*
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export type Username = Lowercase<string>;
+
+export function makeUsernameLowercase(username: string): Username {
+  return username.toLowerCase() as Username;
+}