Use HTML5 datetime picker 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%...

The downtime is, that this won't allow relative inputs, currently the field allows:

You have the following options:

• Leave the field blank. In this case, the role will never expire (since no default duration is configured for this role).
• Enter a date and time in the format YYYY-MM-DD HH:MM:SS. The role will expire after this time.
• Use a relative time, for example: '1 day', '2 months', '1 year' or '3 years'.

Perhaps we could keep the "input" type, but allow a HTML5 datetime select optionally?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet