feat: add SPDX headers

[luka-nextcloud]

Resolves: #1325

[juliusknorr]

We also need:

• Add a github action to check this https://github.com/nextcloud/.github/blob/master/workflow-templates/reuse.yml
• Make sure that the check passes
• Add an AUTHORS file (see chore: Add SPDX headers text#5738)
• Have a reuse config file for cases where we cannot define the license in the headers (see https://github.com/nextcloud/whiteboard/pull/69/files)

[AndyScherzinger]

Changes look good to me reuse/spdx-wise. Just 2 minor comments else 👍