Review Board (apparently?) does not support teams that depend on UTC timestamps.  This is mostly because of the use of `datetime.now` as the default value for timestamp database entries.  Additionally, the default `reviewboard.settings:TIME_ZONE` value being set to 'US/Pacific' probably isn't helping :)

This patch replaces all instances of `datetime.now` with `datetime.utcnow`.

It should be noted that all attempts to get Review Board to use UTC short of applying this patch were unsuccessful, including but not limited to:

    * setting TIME_ZONE to UTC in settings_local
    * running the Review Board server process in an env with both TZ="GST-0" and TZ="UTC"
    * hoping for magic

Ran full regression suite.
Using it in production!  A patched version of Review Board has been in use by an international team that uses UTC for all timestamps at this url:

    http://reviews.meatballhat.com

I'd be happy to provide read access to the database if doing so would be helpful.