[Feature] Reduce dependence on beta.freeso.org

[ItsSim]

The idea is to gradually reduce dependence on beta.freeso.org for downloads and installations, so that the launcher can install and run the game on its own by just querying the GitHub repository.

Obviously, features like the "top lots API" will still depend on beta.freeso.org, but these aren't features that impact installation or launch.

A starting point I believe would be to add sort of a "downloads.json" to the GitHub release files with all the available download mirrors, so that the launcher can directly GET them from this repo. Would be better for maintainability (since I currently need to maintain the separate LauncherResource API), and also useful in case beta.freeso.org suffers any downtime.

Possible issues:

• Clients not being to access GitHub for whatever reason (no known cases?)
• GitHub downtime (not likely)
• GitHub request rate limits - since these are a thing, the mirrors should only be fetched during launcher initialization (every time the launcher is opened, not via a timer).