Fix timezone handling for Flot graphs.

The leading principle is that Javascript should do no timezone
manipulation on time data from the past, because that would mean that
the timezone of events recorded in the past depends on things like the
_current_ summer / winter time setting. A graph of the same data
should show the same information regardless of when it is viewed.

In Lizard we internally work with UTC datetimes as much as possible,
and convert these times to the site's timezone (usually
Europe/Amsterdam) right before handing them to Javascript.