auth: Integrate suggestions by @davidmehren

Add number type alias TimestampMillis
Remove solved ToDos
Change AuthToken and AuthTokenDto to use Date
Rename authService unit tests

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

src/utils/timestamp.ts

@@ -0,0 +1,7 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+export type TimestampMillis = number;