Improve timezones selector with optgroups

Problem/Motivation

When selecting a timezone you have to choose from a list of 422 items. Quickly finding an entry in this list can be difficult as you have to look through options with the same region and typing city names does not work.

Proposed resolution

Group the options by region, listing the city so that typing will select the appropriate entry.

User interface changes

The list of timezones in the widget will change to be a list of cities, and not timezone names.

Before:

After:

Remaining tasks

• Needs Before and after Screenshots. DONE, comments #2 and #18.
• Needs Accessibility Review. DONE, comment #19
• Needs Usability Review DONE, comment #34
• Approve API addition. Needs confirmation that it is backwards-compatible.
• Needs change record for the API addition.

API changes

Adds an optional new $grouped = FALSE parameter to system_time_zones(). If true, the timezones are returned with an additional level of nesting, grouped by continent.

Data model changes

None.