Invalidate client cache after some time

[MaSch0212]

Currently, when installing the App on a mobile device, the app state is persisted as long as the app is still in memory.
As admins change data, the changes are not refreshed in the other users apps. This will be finally addressed with #26. But as this feature is quite large, maybe we can just invalidate the client cache after some time has passed, or the app gets refocused.

So maybe when opening the app, after is has been put in the background, the client removed cached data, so they get reloaded. Will need to check what the current state is (maybe you care currently editing an event), because you do not want to remove the whole state.