fix: make export use correct time zone

[condyl]

This pull request updates the calendar export functionality to generate more accurate and standards-compliant ICS files, with a particular focus on correct timezone handling for events. The changes ensure that exported calendar events use the "America/Toronto" timezone explicitly, include proper timezone definitions, and generate timestamps in the correct format for both local and UTC times.

ICS file format and timezone improvements:

• Added a detailed VTIMEZONE definition for "America/Toronto" to the calendar export, ensuring compatibility with calendar applications and correct handling of daylight saving time.
• Introduced the ICS_TIMEZONE constant and updated event start/end times to use TZID=America/Toronto and generate timestamps in the specified timezone, improving the accuracy of event times in exported calendars.
• Changed the recurrence rule's UNTIL date and the event DTSTAMP to use UTC-formatted timestamps for compliance with ICS standards.

Pink-ish color: new
Yellow: old (shifted forwards an hour or completely messed up with the 3 hour classes