Switch to github actions

[dequis]

A few months ago I hired an intern to move to github actions. That work is probably finished in the "gh-actions-finished" branch of this repo, and should be reviewed and merged (squashed, likely).

Sadly, I don't really have time for this, so it's been waiting there for months.

Goals:

• Don't use travis
• Have something that kinda works for building packages on OBS (commits to master trigger builds there)
  • Probably should do something about jgeboski.github.io, jgeboski hasn't been involved in this stuff in years
• Build win32 packages and have them as artifacts
• Bonus: release workflows? Something to attach binaries and tarballs to github releases. The current release process sucks.

Somewhat related to #516

[dequis]

Oh right, PR #509 exists. This ticket is basically "review that PR" really.