Create self-update-mechanism for website apks
- in order to provide a better support of ungoogled devices it would be nice to implement an update mechanism that checks our download space on leap.se for new apks on regular basis, e.g. every 2 days. This way we don't depend so much on F-Droid.
- The update mechanism should be only available for web-downloaded apks. F-Droid users should stay with F-Droid, Google Play users with GPlay.
- Nice to have: the downloaded apk will be verified
- consider: check for updates only if the user has vpn running?