Allow setting time granularity in datetime and datetime_range

Problem/Motivation

For datetime and datetime_range values, there is often no need to be precise down to the second, but the form display widget requires that the seconds be entered.

Proposed resolution

Add a granularity control that hides the seconds from the form, defaulting to zero.

Remaining tasks

Implement.

User interface changes

Site builders may choose to hide the seconds component of the widget. Users will not see seconds if this option is chosen.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet