Rebuild Content Update subscription functionality

Tow alternatives to implement content subscription in D10:

• DANSE
  https://www.drupal.org/project/danse →
• Message and Message Subscribe
  https://www.drupal.org/project/message →
  https://www.drupal.org/project/message_subscribe →

Message subscribe example is bugged right now. Let's go with DANSE

DANSE is working.It requires from to be enabled and executed regularly. Now we have to add the push framework to be able to provide Notifications by E-Mail. Slack may be added later.

/requires from to|requires cron to/

Added pushframework, pf_mail and pf_slack and initial config export in #3422866: Add push framework to codebase →