refactor: move permissions decorator and guard into permissions directory

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

backend/src/permissions/permissions.decorator.ts

@@ -0,0 +1,17 @@
+/*
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { CustomDecorator, SetMetadata } from '@nestjs/common';
+
+import { Permission } from './permissions.enum';
+
+/**
+ * This decorator gathers the {@link Permission Permission} a user must hold for the {@link PermissionsGuard}
+ * @param permissions - an array of permissions. In practice this should always contain exactly one {@link Permission}
+ * @constructor
+ */
+// eslint-disable-next-line func-style,@typescript-eslint/naming-convention
+export const Permissions = (...permissions: Permission[]): CustomDecorator =>
+  SetMetadata('permissions', permissions);