Add timezone selection

## Problem

Public events or meetings are often given in timezones that are not local to users. Instead of having to do the math themselves, we should let them enter the time in the local timezone for the event and assign the appropriate timezone.

## Proposal

Add a way to select the time zone of an event. This can take many forms, a combo box, a search box, or we could even copy the popover from Date & Time settings.

I did a bit of work that can help set this up in #653, including adding a timezone label and a timezone property in the `InfoPanel`.

## Prior Art

Google Calendar:
Fantastical (old screenshot, but it's stayed about the same):