Mac Desktop App: set up certificate on Github Actions and automate Mac Installer build process

[ryanw-mobile]

Wish to assign a P1 priority because as we spend less time on this project, eventually, we'd forget how to set up Mac desktop app release - especially if our local settings are gone.

Let Github Actions build the MacOS installer would typically take an extra 15 to 30 minutes for the release process to complete, but at least it's automated and we don't have to worry about local computer setup.

When this happens, we will also publish a blog post concluding the process.