Add per-instance datetime timezone storage

Problem/Motivation

This solves the problem when creating meeting entities to describe meetings held at various locations across the world, the intended time zone needs to specified for each meeting by the editor, and that preference needs to be stored with the date/time.

A few more reasons for per-date settings.

a) We have to handle fields with existing values that will have a NULL stored time zone. And having this setting, which will then be ignored most of the time, is slightly misleading for site builders. It would make it more bluntly obvious how things worked if you have the additional checkbox for "If a preferred time zone is stored, use it instead of the default". This drives home the idea there are two different things at work here, a default and something that can override it per-date.

b) This would allow site builders to render a time in its canonical (venue) time zone and sometimes in the time zone of the user.

c) This would allow site builders to hide the time zone selector on a "basic" form but have it exposed on an "advanced" form for superusers.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet