Better support of times & timezones

- use `LocalDateTime` instead of `DateTime' for client facing timestamps.  these values are just a timestamp without any timezone (conference fields: registration start & end times, event start & end times, early registration cutoff
- upgrade to latest sql2o version which adds support for `LocalDateTime`
- use separate Json serializers for `DateTime` and `LocalDateTime`
Actually, that's not true.  Both those methods actually returned a UTC DateTime, it was simply calling two different methods which do the same thing.  I eliminated the duplicated methods