Timezone support for datetime fields

**Is your feature request related to a problem? Please describe.**

I'm using this CMS as a backend for a small website built via SSG so the owner can maintain it themselves. There is an "events" collection and it's important that they can specify the timezone for an event.

**Describe the solution you'd like**

A timezone field for the datetime picker.

**Describe alternatives you've considered**

I could create a select with a few timezones in it and then recalculate datetime value relative to that.

**Additional context**

Start by tracing the datetime picker used for the events collection and review how its value is handled for the SSG-backed site. Define the expected timezone-selection behavior and verify that an editor can specify an event timezone; the issue names no files or tests.

Written by the indexing model from the issue text.