Delete my entire account button in app

Users report that there's no option to delete our account with an easy method.

to delete an account, we should provide e.g. a clickable ListTile in the Settings-Menu to delete your own account.

This will decrease the overhead for backend.

@rapl42 I think this would require a larger function, but only 1 route. we should discuss this.

@rapl42 is already working on it, a function can be provided/accessible in the future.