Use datepicker for expiration

Problem/Motivation

The expiration date input should be simplified by using a date picker, like other areas in Drupal do.

#type => 'datetime'

See https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Datetime%...

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet