Remind users about version upgrade

[jannisborn]

When starting the app and there is a new version available, user should see a popup with three options:

• update -> lead to appstore
• Not Now -> Message dissappears
• No thanks -> message will not re-appear until new version is released

Instead of coding manually, probably the best is to use Siren: https://github.com/ArtSabintsev/Siren#implementation-examples

[NinaWie]

Started implementation on branch include_siren