Ensure that all original source files contain SPDX headers

Ref #343 (closed)

It's a big change, I know, as it touches virtually every file in the codebase.

To summarize, it adds a SPDX header to our files, with the license AGPL-3.0-or-later, and onwner Lightmeter <hello@lightmeter.io>.

Other third party files are licensed accordingly.

This MR also updates the documentation about how to handle the licensing on new files.

Finally, it sets up a gitlab job that check if the codebase is compliant with the Reuse tool and all SPDX files are valid.