Update SPDX headers
We started using SPDX headers a while ago to track copyright changes on individual files. These headers look like this:
SPDX-FileCopyrightText: 2019-2022 Katharina Fey <kookie@spacekookie.de>
SPDX-FileCopyrightText: 2019-2022 Yureka Lilian <yuka@yuka.dev>
SPDX-License-Identifier: AGPL-3.0-or-later WITH LicenseRef-AppStore
Unfortunately many files in the repository don't yet have SPDX headers, and those that do have outdated ones. Many contributions are hard to track for files because of the long history of renames and refactoring that has moved code around over the years.
This is a tracking issue to tackle this problem, and to give people who have contributed code to various bits around the codebase the recognition and copyright that they are owed.