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`
For the majority of records, there is no issue.  Dropping the the timezone will leave us with the correct timestamp.  I'll record a back-up of the table and do a comparison before and after.. manually updating any records that need it.  There are only a few live conferences, so this will be easy.