Implement more robust date representation in traits table

[gsrohde]

• [x] Add a time_zone column to the sites table.
• [x] Populate time_zone column for sites that fall within a single time zone.
• [ ] Clean up the traits table, consolidating all date information into the date, dateloc, and timeloc columns.
• [ ] Deploy updated BETYdb Web interface that is consistent with new date representation schema.
• [ ] Eliminate columns time, date_year, date_month, date_day, time_hour, time_minute from the traits table.

This issue relates to the issues discussed in #347, #357, and most extensively at #356.