chore: add user relation enum

this enum is used to specify which relation of the user object should be populated.

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

src/users/user-relation.enum.ts

@@ -0,0 +1,10 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export enum UserRelationEnum {
+  AUTHTOKENS = 'authTokens',
+  IDENTITIES = 'identities',
+}