Set TIMEZONE to UTC by default

 * helps prevent bugs like #893 (461c5f5 made non-TIMEZONE people break,
   it was the reverse before)
 * could help us get rid of some useless code for testing whether
   TIMEZONE is set (we can happily crash on people that have an invalid
   value there)
 * we assumed UTC (incorrectly called GMT in conf.py.in before) anyways,
   so nothing changes in output
 * config options set to `None` by default suck (we should have a
   singleton named UserIsAnIdiotAndWeShouldCrashHorribly (or something
   like this) and crash whenever any config value is set to it (i.e. we
   should have sane defaults where we can, and required config values
   where we can’t)

Done with @ralsina’s blessing.

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>