Contacts can't communicate if clock difference is too large

If a device's clock is very inaccurate (e.g. because the user has adjusted the clock when travelling, rather than changing the timezone) then it's possible to add a contact, but not possible to communicate with the contact subsequently because the devices disagree about which rotation period they're in.

We should check the clock difference when adding a contact and if the difference is excessive, display the time and timezone and warn the users to check that they're using the same timezone. We may also need to make the maximum expected clock difference much larger (on the order of 24 hours rather than an hour), as contacts may change their clocks after adding each other.