Support for auto-updating APK

[micressor]

The .apk files offered directly on github should have support for security relevant automatic APK updates.

Links:

• Signal Messenger: Support for website distribution build with auto-updating APK
• #152

[dipunm]

1. Why is this related to f-droid?
2. Why should manually downloaded packages auto update. How can a user vet that the update is security only? What if the auto update is compromised? It only takes one malicious update to steal funds.