fix: Move content into to frontend directory

Doing this BEFORE the merge prevents a lot of merge conflicts.

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/src/redux/user/reducers.ts

@@ -0,0 +1,23 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { Reducer } from 'redux'
+import type { OptionalUserState, UserActions } from './types'
+import { UserActionType } from './types'
+
+export const UserReducer: Reducer<OptionalUserState, UserActions> = (
+  state: OptionalUserState = null,
+  action: UserActions
+) => {
+  switch (action.type) {
+    case UserActionType.SET_USER:
+      return action.state
+    case UserActionType.CLEAR_USER:
+      return null
+    default:
+      return state
+  }
+}