datetime2 component migration

To prepare for the Popover changes coming in Blueprint v5.0, we're providing "v2" variants of @blueprintjs/datetime and @blueprintjs/timezone components which use Popover2 instead of Popover. Migrate to these now in order to ensure a smooth migration path.

These components are available in the new @blueprintjs/datetime2 package:

DateInput2

Replacement for DateInput.

TODO(adahiya)

TimezoneSelect

Replacement for TimezonePicker.

TODO(adahiya)