Fix icalendar timezones

[leoleoasd]

fixes #7456

Currently, icalendar module will treat all times as local timezones. Also, TZID in DTSTART of icalendar events are ignored, resulting more confusing result (as events with local timezones are displayed correctly but events with other TZID are shifted).
This PR fixes both these problems.

1. use toJSDate().toISOString() as ical.js's toString() method don't include timezones other than UTC or local:

https://github.com/kewisch/ical.js/blob/b8157d34d61ce15843ea55cd05d9028414ccfdff/lib/ical/time.js#L1055-L1071

2. add timezone info to ical.js

[acerix]

Please submit this ticket under Issues with a runnable, stripped-down demonstration of the bug, then link this PR to it. I found a similar issue #7364 but it sounds like not the same as what this is fixing.

[leoleoasd]

I discovered my problem and write this fix, this MAY fix #7364 as well but I havn't tested.

[leoleoasd]

Before:
(Event should start at 12:00 and end at 13:00 in china timezone; I used devtools to change the timezone to china)

(Event should start at 00:09 and end at 01:09 in ET; No devtool option used so it should my local TZ which is UTC-4)

After:

[leoleoasd]

Any update? @acerix @arshaw