Fix and enable automatic build / publish

[kamilzyla]

Problem

The release process is currently manual. After creating a new release on GitHub, we need to manually build Mbaza on 3 different platforms (Windows, macOS, Linux) and upload the results. It takes same time and invites mistakes, like forgetting to download and place the model files in assets/models.

Goal

It should be possible to automate the build / publish process by leveraging GitHub Actions. We already have .github/workflows/publish.yml which came with the electron-react-boilerplate, but there were some problems with it. Additional challenges:

1. The Windows binaries need code signing (the certificate is on 1Password). It would be great to automate this as well.
2. We'd need some way to download the models prior to build.