Util: add default timezone to CrabEventFilter.

This class now has a static default_timezone variable
and a class method set_default_timezone for manipulating
it.  This allows all times displayed in notifcations and
on the web interface to be formatted consistently.
(Previously raw database times were shown if the job lacked
a timezone.)
Server: default timezone to notification timezone.

The CrabEventFilter default timezone is now set to the
timezone configured in the notification section of the
server configuration.  This is probably more likely
to be useful than always defaulting to UTC, but a second
configuration parameter could be added if a need
arises to configure the daily notification timezone
separately from the default display timezone for
notifications and the web interface.
Web: add checks to update of Crab IDs.

The form now displays an empty box if the ID is not set.
When attempting to perform the update, the server now
checks that the ID is not blank and does not already
exist.