Remove all timezones, use UTC throughout

[denny]

This would be intended as a rather heavy-handed but temporary step 1 on the way to adding some timezone support that actually works. Right now I'm pretty sure this app will have different bugs in different countries at different times of the year, because Rails is doing magic in some places and not others.

(Eventual solution needs to support sites with users in multiple countries/timezones... but for now just making things consistent would be good.)