Calendar: Better management of timezones

• /e/ version: 2.6.3-t
• Device model(s): Google Pixel 5

I got confused with the time of several events that I created when I was on a different timezone to my hometown. The events, flights most of all, where supposed to have the hometown time.

Therefore, I propose to add a dialog to alert of and manage timezones in the calendar:

When the timezone got changed in the device, calendar should display a dialog similar to this:

The timezone of your device was changed. Now you are using [NEWTIMEZONE], [NUMBER] hours more/less than your previous timezone.

How do you want to manage time on your events from now on?

• Use the previous timezone ([TIMEZONE_CODE])
• Use the new timezone ([TIMEZONE_CODE])
• Ask every time I create an event