Allow to schedule blökkli drafts

Problem/Motivation

As a content editor I would like to schedule drafts so that I can have a published version of a content already that gets replaced by a future draft

Proposed resolution

Keep it simple without relying on revisions, content moderation

Idea
https://www.drupal.org/project/scheduler/ →

Schedule blökkli draft
* In blökkli, add an option - "Schedule publishing"

Inform users about scheduled blökkli draft
* When the blökkli state is scheduled for publishing
* * while editing in blökkli, a message appears "This content is scheduled to be published DATE"
* * while editing in the backend, the same message appears

Let editors manage scheduled drafts
* As editor I would like to see the scheduled drafts in the overview that I am used to. Idea is to extend the list (admin/content/scheduled) to also show scheduled blökkli drafts

Publish scheduled blökkli draft
* When the scheduler kicks in (scheduled blökkli draft is executed at the scheduled time)
* * The scheduler module blökkli integration publishes the draft state of the content

Remaining tasks

User interface changes

API changes

Data model changes