Summary:
Addresses request https://github.com/gristlabs/grist-core/issues/443
The bootstrap-datepicker used for the Date/DateTime dropdown calendar does
support a number of locales. This diff loads them on-demand, if available,
based on the document's locale setting.
Also:
- Improves NewBaseEditor typings, reduces some casts, adds comments.
- Converts DateEditor and DateTimeEditor to typescript.
- Moves DateEditor nbrowser test to core.
Test Plan: Added a test case for locales to DateEditor test.
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D4335
