Migrate Sparkle builds off S3

[lucasderraugh]

We currently host GitUp app zip files on S3 which @swisspol still graciously hosts for this project. This issue tracks moving off S3 and simply hosting via GitHub. The proposal is as follows:

• Create a continuous and stable appcast.xml entry within this repository
• These new appcasts will point to the corresponding GitHub releases made directly on this repository. GitHub releases becomes the new host of the final zip download
• Update the app to point to these appcast entries rather than the S3 ones. Push this update to S3 so everyone has ample time to adopt the new appcast urls in the app
• Update the website to point to the latest release on GitHub
• Drop S3

This process may be slightly manual at first, but in time will become more automated.

[lucasderraugh]

I think this will also involve signing with EdDSA keys so I can move to signing with my own certificates. https://github.com/sparkle-project/Sparkle/issues/1283