Parse events' start/end date as UTC

When rendering the iCalendar for my site, the dates are rendered in London's timezone, although they're the native date.

This is not an issue for most of my events, but when I attend events that aren't in the same timezone as I am, this leads to inconsistent events in my calendar.

This forces them to be UTC, so the site should correctly render them.

Closes #242 (closed).