Enable CI using GitHub Actions

[CelticMinstrel]

This is a pull request so I can more easily sort through it and figure out how things should be squashed together, because while the bulk of it could be compressed, there are at least a few things that definitely shouldn't be.

[NQNStudios]

Apparently GitHub actions can upload automatic releases. This action here seems like a popular way to do so. Maybe uploading "nightly" builds could become part of the CI.

[CelticMinstrel]

It's a good idea, but at this point I'd rather get the CI to even just function. Uploading artifacts can come once it works.

This was working back when I worked on it. It likely doesn't work anymore due to dropping legacy systems, but we'll see…

[CelticMinstrel]

Merged manually.