Allow end date to be optional

Problem/Motivation

The 7.x Date module allowed the field to have an optional or required end date. D8 always requires the end date.

Proposed resolution

Make end_value in DateRangeItem optional, add validation constraint via configuration.

Remaining tasks

1. If the start date is missing, stop deleting the end date on saving the form. (See #84.)
2. If missing, indicate that the end date is unknown/unspecified on display. (See #84.)

User interface changes

API changes

Data model changes